Here is the shortcut before the reasoning: splash the patch with water and watch for a few seconds. Moss is a plant, so its little leaves soften, unfurl and turn a clean green almost at once. A lichen barely stirs, because it has no leaves to open and no green tissue of its own to brighten. Most of what people point at on a branch or a coping stone and call moss is one of these two, and once you can part them the rest of naming comes far more easily.
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Two organisms built on different plans
A moss is a single green plant, a bryophyte, with its own tiny stems and leaves. It makes its food by photosynthesis and drinks in water and minerals across its whole surface rather than through roots, as set out in whether mosses have roots. A lichen is not one organism at all. It is a partnership, a fungus sheltering a colony of algae or cyanobacteria, the fungus supplying the body and grip while the algal partner makes sugars in the light. So when you compare the two you are holding a plant in one hand and, in the other, a fungus that has taken up farming. Nearly every visible clue below flows from that single fact.
The tests that settle it outdoors
Wetting comes first and is quickest. Give the patch a drink and moss responds within a minute, leaves spreading and colour deepening; a lichen looks much the same wet or dry, perhaps a shade darker but never leafy. Reach for a hand lens next. Moss shows unmistakable little leaves, usually spiralling up a visible stem, and you can tease a cushion into separate shoots. A lichen has no leaves anywhere: it forms a flat crust welded to the surface, or a set of leathery lobes, or a branching miniature shrub, and it is the same colour and texture right through. Colour itself is telling, since moss is green whenever it is alive and moist, whereas lichens run to sage grey, mustard yellow, rust, black and chalk white and keep that colour in all weathers. Look last at how each fruits. A moss lifts spore capsules on fine stalks; a lichen bears little discs or cups pressed against its body, never on a slender stem.
When the name points the wrong way
English has spent centuries calling anything soft and shaggy a moss, and a good deal of it is nothing of the sort. Reindeer moss, the pale springy stuff of heath and tundra, is a lichen of the genus Cladonia, a muddle untangled in reindeer moss and reindeer lichen. Oak moss, gathered for perfumery, is the lichen Evernia prunastri. Iceland moss, brewed as an old chest remedy, is another lichen again. Spanish moss draping the oaks of the American South is neither: it is a flowering plant related to the pineapple. Even Irish moss and sea moss belong to the seaweeds. If a name has stuck to something grey, crusty or hanging in curtains, treat the word with suspicion and run the tests above.
Sharing the same bark and stone
The confusion is worse because the two crowd onto the same surfaces. On a tree trunk the soft green fuzz is almost always moss, while the flat grey-green rosettes and the scattered crusty blotches beside it are lichens; neither one feeds on the tree or does it any harm, a point I make at greater length in moss on trees. Gravestones and old walls tell the same story: the hoary little domes are usually moss, and the flat maps of grey, ochre and rust spreading over the flat faces are crustose lichens gripping the bare stone. Roofs, fence posts and tarmac all carry the mixture. Once you expect both, you stop forcing every patch into one box and start reading the surface as the shared ground it really is.
Why the difference earns your attention
Beyond satisfying curiosity, it changes what you can do. If your aim is to grow a green covering, only moss will oblige, taking to transplanting and to a blended slurry within a season or two; lichens grow so slowly, often a millimetre or two a year, and resent moving so thoroughly that cultivating them is close to hopeless. For identification more broadly, ruling out lichen at the first glance clears away half the possibilities before you have even reached for a key, which is why I always start there before the finer work in how to identify moss. And there is a third green thing that trips people on wet paving, the formless film that is neither plant nor partnership, kept separate in moss and algae. Learn these three apart and a damp wall stops being a blur and turns into something you can actually read.