Moss is a small leafy plant; algae is a formless green film with no leaves or stems at all. That single distinction settles most cases: if you can tease it into tiny leafy shoots it is moss, and if it smears into a streak of green with nothing structured left behind it is algae. Both love damp and shade, which is why they turn up in the same places and get muddled, but they are different organisms and each wants a different response.
The quick test
Crouch down, look closely, and run a finger across the green. Moss holds together as tiny cushions or a soft carpet made of little leafy stems, often with fine stalks carrying capsules above the green; pick at it and you can lift leafy shoots you could tease apart. Algae has no such structure. It is a film, a slime or a powdery bloom that wipes into a smear and leaves nothing leafy behind. A hand lens turns a hunch into certainty in seconds, and once you have checked a few patches the answer comes at a glance. This is the same field test that separates moss from its other lookalikes, covered in moss, lichen, liverwort or algae.
Why they are so different
Moss is a true plant, a bryophyte, with stems and leaves usually a single cell thick, anchored by fine rhizoids and spreading by spores and fragments. Algae is a much simpler thing: a loose growth of single-celled or filamentous organisms with no leaves, stems or roots, closer in organisation to pond scum than to a plant. That is why moss forms a structured, springy green you can garden with, while algae forms a flat film that colonises any surface staying reliably wet. The marimo "moss ball" sold for aquariums is in fact a ball of alga rather than a moss at all, a mix-up untangled in the marimo moss ball.
What each one tells you about a surface
The two are readable. Algae marks a surface that is simply too wet and too shaded, with water sitting long enough for a film to establish; it is a report on drainage and light more than a plant to be fought. Moss says much the same about damp and shade, but it also needs a surface it can grip and a little more stability, so it favours joints, mortar, bark and soil over smooth wet glass or plastic. On a patio you often meet both at once: a slick algal film pooling in the low, shaded spots, and moss cushions seated in the joints where they can hold. Reading which is which tells you what to change.
Controlling algae
Because algae is really a symptom of a surface staying wet in the shade, the durable fix is to dry it and light it. Clear overhanging growth to let sun and air reach the spot, sort out drainage so water runs off rather than pooling, and the film loses its footing. For the algae already there, a stiff brush and plain water shift most of it from paving, decking and render, and a pressure washer makes short work of a large area, though it roughens the surface and can invite quicker regrowth. Where you want a chemical helper, a patio or path cleaner will do it, but read it as a broad biocide: most kill moss and plants as readily as algae, so keep them off borders and ponds. On decking and around water, choose the gentlest option and rinse well.
Controlling moss, or keeping it
Moss is a choice more often than algae is. In a joint, on a shaded wall or across a north-facing roof it may be a welcome softening you would do well to leave, and much of what people reach for the moss killer over is doing no harm at all. Where it genuinely needs to go, on a slippery step or a path, the approach differs from algae: moss is lifted or scraped rather than simply wiped, since it grips by rhizoids, and it returns unless the damp shade that admitted it is eased. There is a fuller account in removing moss, and the case for living with it in moss on walls and brickwork.
When you want to keep the moss but lose the algae
This is the awkward case, and it is common on old paving and stone where a handsome moss shares the ground with a slick algal film. Blanket treatments will not tell the two apart, so work by hand. Brush or scrape the algal film from the open, slippery surfaces where it is a hazard, and leave the moss cushions in the joints and shaded corners where they do no harm and look their best. If you must use a cleaner, paint or spot-spray it onto the algae alone rather than drenching the whole area. Improving light and run-off helps here too, since it sets algae back hardest while established moss, better anchored and more drought-tolerant, shrugs off a drier spell and recovers.
In the aquarium
The same confusion follows moss into the fish tank, where wanted aquarium mosses and nuisance algae grow side by side. Here the leafy structure of a Java or Christmas moss is easy to tell from the hair, film and beard algae that coat glass and hardscape, but the two respond to the same levers. Algae blooms on excess light and nutrients with too few plants taking them up, so the cure is a fuller planting, a shorter or dimmer light period and steady maintenance rather than a scrub. A healthy stand of moss is part of that cure, competing for the nutrients algae would otherwise seize; the practical side is covered in aquarium mosses.
Common questions
What is the difference between moss and algae?
Moss is a small green plant with tiny leaves arranged around a stem, so it looks leafy and can be teased apart into shoots. Algae has no leaves, stems or structure at all; it is a formless green film or slime you can wipe off in a single smear. Both like damp and shade, but they are different organisms and want different treatment.
Is the green on my patio moss or algae?
Look closely and try to smear it. If it wipes into a green streak with nothing leafy left behind, it is algae. If it holds together as tiny leafy cushions or a soft carpet, it is moss. On paving you often get both at once, a slippery algal film in the wettest spots and moss cushions in the joints.
Does killing algae kill moss too?
It can. Many patio and path cleaners are broad biocides that hit both, so if you want to keep a moss you value, spot-treat the algae rather than drenching the whole surface, or clear the algae by hand. Improving light and drainage discourages algae while leaving established moss largely alone.
Which is more slippery, moss or algae?
Algae, by a wide margin. A wet algal film on smooth paving or decking is genuinely treacherous, whereas moss gives more grip underfoot and is slippery mainly when a thick cushion sits on a hard, wet surface. If a path has turned dangerous, algae is usually the culprit.