Moss vs Algae vs Lichen: What’s the Difference?

That green stuff on your patio. Is it moss? Algae? Something else? Does it even matter?

The short answer: yes, it does matter. While all three can make your outdoor surfaces look neglected and slippery, they’re actually very different organisms. Understanding what you’re dealing with helps you choose the right treatment and set realistic expectations for removal.

Let’s break down the differences.

Moss: The Fluffy One

Moss is probably what most people picture when they think of green growth on patios and paths. It’s a small, non-flowering plant that forms soft, cushion-like clumps.

You can identify moss by its texture. Run your hand over it and it feels soft and spongy, almost velvety. Look closely and you’ll see tiny leaf-like structures. When it’s wet, moss is bright green and springy. When it’s dry, it goes darker and feels crispy, but it’s not dead. It’s just dormant, waiting for the next rain.

Moss sits on top of surfaces rather than bonding to them. It holds onto rough textures and gaps but doesn’t actually penetrate into the material beneath. This makes it relatively easy to remove once it’s been killed by treatment.

Moss thrives in damp, shaded conditions. It spreads through spores that float through the air, which is why it seems to appear from nowhere. If you’ve got a shaded patio that stays damp, moss is almost inevitable in the UK climate.

Common places you’ll find moss include patios, paths, decking, roofs, and lawns.

Algae: The Slimy One

Algae is much simpler than moss. It’s not a plant at all, but a group of organisms that photosynthesise like plants do. On outdoor surfaces, it typically appears as a thin, slimy green film.

The key identifier for algae is texture. Where moss is fluffy and three-dimensional, algae is flat and slippery. When wet, an algae-covered surface feels like it’s been coated in slime. This is what makes algae-covered patios and paths so dangerously slippery.

Algae doesn’t have leaves or structures you can see with the naked eye. It’s just a green (sometimes brown or black) coating that stains the surface. Unlike moss, you can’t peel algae off in clumps. It’s more like a stain than a growth.

Algae grows incredibly fast in the right conditions. A surface can go from clean to green in just a few weeks during mild, damp weather. It loves moisture even more than moss does, and you’ll often find it worst in areas where water sits or runs regularly.

Common problem areas for algae include patios, driveways, paths, rendered walls (especially below gutters), and fencing.

Lichen: The Crusty One

Lichen is the odd one out. It’s not a plant, and it’s not a single organism. It’s actually a partnership between a fungus and algae (or cyanobacteria), living together in a mutually beneficial relationship.

You can identify lichen by its appearance. It forms flat, crusty patches that are firmly attached to the surface. The most common type on UK patios is black spot lichen, which looks like someone has flicked black or dark grey paint at your paving. Other lichens can be grey, green, orange, or yellow.

The texture of lichen is dry and scaly, not soft like moss or slimy like algae. Try to scrape it off and you’ll find it’s bonded incredibly tightly to the stone. Unlike moss, lichen actually grows into the surface material, sending tiny root-like structures into the pores of the stone.

Lichen grows extremely slowly, sometimes just a few millimetres per year. But it’s also extremely long-lived. Those black spots on your patio could be decades old.

Common surfaces where lichen appears include natural stone patios, concrete, gravestones, walls, and rocks.

Quick Identification Guide

Still not sure what you’ve got? Here’s a quick test:

Touch it. If it’s soft and spongy, it’s moss. If it’s slimy and slippery, it’s algae. If it’s dry and crusty, it’s lichen.

Try to lift it. Moss will peel away in clumps. Algae won’t lift at all because it’s just a film. Lichen is firmly stuck and won’t budge without serious effort.

Look at the colour. Bright green and fluffy is moss. Thin green slime is algae. Black, grey, or other coloured crusty patches are lichen.

Check the growth pattern. Moss forms irregular cushions and mounds. Algae coats surfaces evenly. Lichen forms distinct circular or irregular patches with defined edges.

Why Identification Matters for Treatment

All three respond to similar treatments, but they behave differently:

Moss is the easiest to deal with. It responds quickly to treatment (usually dying within a week or two) and brushes off easily once dead. Most moss killers show results fast.

Algae is also fairly easy to treat, but because it grows so quickly, it can return faster than moss if conditions remain favourable. You may need more frequent preventative treatments in problem areas.

Lichen is the most stubborn. It takes longer to respond to treatment (sometimes three to four weeks before you see colour change) and requires more effort to remove even when dead. Its protective outer layer means you need a treatment that can penetrate properly.

The good news is that a quality moss, mould and algae killer will handle all three. For help choosing the right product, see our independent comparison of the best moss killers in the UK. You don’t necessarily need to identify exactly what you’ve got before treating. But understanding the differences helps you know what to expect and when to consider a second application.

Can You Have All Three at Once?

Absolutely. It’s common to find moss, algae, and lichen all growing on the same patio, especially if conditions are right (shaded, damp, and left untreated for a while).

In these cases, treat the whole area and expect mixed results. The moss and algae will die and clear relatively quickly. The lichen will take longer but will eventually succumb too. You might need a follow-up treatment for stubborn lichen patches.

Our complete guide to moss, mould and algae removal covers treatment strategies for all three in more detail.

Prevention Is the Same for All Three

Whatever combination you’re dealing with, prevention follows the same principles. Improve drainage so surfaces dry faster. Increase sunlight by trimming back overhanging vegetation. Keep surfaces clean and free of organic debris. Apply preventative treatments once or twice a year to vulnerable areas.

Get these basics right and you’ll spend a lot less time identifying and treating green (and black) growth on your outdoor surfaces.

Frequently Asked Questions

Is moss or algae more dangerous on a patio?

Algae is more dangerous because it forms a slippery film that becomes treacherous when wet. Moss can also be slippery but tends to provide more grip than algae. Both should be treated promptly on paths, steps, and patios where people walk regularly.

Can one product kill moss, algae, and lichen?

Yes. Products based on benzalkonium chloride are effective against all three. The difference is timing: moss dies within one to two weeks, algae responds within days, and lichen can take three to four weeks to show visible results. A single application treats whatever combination you have.

Why is lichen so much harder to remove than moss?

Lichen grows into the surface material rather than sitting on top of it. It sends tiny root-like structures into the pores of stone, concrete, and brick, creating an extremely strong bond. It also has a tough protective outer layer that slows penetration of treatments. Patience and a second application are sometimes needed.

Does pressure washing remove lichen?

Pressure washing can remove some lichen from the surface, but it rarely gets all of it because lichen bonds into the material itself. Chemical treatment first, followed by pressure washing to remove dead growth, gives much better results. On delicate surfaces like Indian sandstone, chemical treatment alone is the safer option.

How do I know if the black spots on my patio are lichen or dirt?

Try scraping a spot with a fingernail or plastic scraper. Dirt lifts off relatively easily. Lichen is firmly bonded to the surface and resists scraping. Lichen patches also tend to have defined edges and a slightly raised, crusty texture that dirt does not have.

Can moss, algae, and lichen damage paving?

Over time, yes. Moss can lift slabs by growing into joints and expanding when wet. Lichen gradually etches into stone surfaces. Algae itself does not cause structural damage but creates a slip hazard and traps moisture against the surface, which can accelerate weathering in freeze-thaw cycles.