Call something mould and you have named a fungus; call it moss and you have named a plant, and the two get by on opposite rules. Which matters the moment you find a dark patch creeping across a wall, a grey fuzz over the compost in a pot, or a bloom on the driftwood in a fresh terrarium and want to know whether to worry. Most of the time a close look decides it in seconds, and knowing which you are dealing with is what tells you whether to act at all.
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A plant and a fungus
Moss is a green plant, a bryophyte, with real if tiny leaves and stems and a full complement of chlorophyll, the green pigment. It feeds itself by photosynthesis, building sugars out of light, water and air, which is why a living, growing moss is always some shade of green. Mould belongs to a separate kingdom altogether, the fungi, and has no chlorophyll at all. Unable to make its own food, it lives by digesting whatever it grows on, be that bread, leather, damp plaster or rotting wood. A mould colony has no leaves; it is a felt of microscopic threads, the hyphae, that spread through the surface and push up dust-fine spores. So although both may appear as a patch on the same damp wall, they sit about as far apart as a fern and a mushroom.
What tells them apart up close
You rarely need more than your eyes and perhaps a hand lens. Moss forms a napped, cushiony or feathery growth in which individual leafy shoots can be picked out; a lens shows unmistakable little plants, ranks of green leaves on a stem. Mould has no such build. It sits as a flat bloom or a fuzzy, cottony or powdery layer that resolves under a lens into threads and specks, never leaves. Colour is the next guide. Living moss reads green, sometimes dull or reddish but recognisably green, and when it dries it browns off, then greens again within minutes of a wetting, a recovery no mould can manage; mould comes in white, grey, sooty black, blue-green or pink and holds that colour wet or dry. Texture and smell round it off, moss soft and springy and smelling of little beyond damp earth, a mould patch often slimy or dusty and carrying the flat, musty note of a shut cupboard.
Where each one turns up, and what it means
Because moss runs on light, it grows where the light is: the outside of a north wall, a shaded roof slope, damp paving, a tired stretch of lawn. It needs the surface to stay moist for long spells, but of itself it does no harm to sound brick or stone, and where it appears it is reporting shade and lingering moisture rather than causing it. Mould leans the other way. It has no need of light and often prefers the dark, so its haunts are the cold corners and stagnant places where damp gathers: bathroom sealant, the reveal around a cold window, the wall behind a wardrobe pressed against an outside gable. Mould indoors is a sign that moisture is sitting somewhere it should not, usually from condensation or a leak, and it deserves to be taken seriously. Sizeable indoor growth can aggravate asthma and allergies in some people, so the sensible course is to find and cure the damp and open up the ventilation, and to get professional advice where the affected area is large or keeps coming back. I grow moss; I am no building surveyor, so read that as a nudge to look properly rather than the last word.
The terrarium mix-up
The muddle crops up indoors too, in terrariums and vivariums, though here all that is at stake is your patience. A newly planted mossy jar very often blooms with a white or greyish fuzz over the wood and any dead leaves in the first weeks, as fungi feed on the fresh organic matter in the warm, still, humid air. It looks alarming and is nearly always harmless and short-lived; a little more airflow, a touch less water and a colony of springtails usually see it off, as covered under moss terrarium troubleshooting. Do not read it as the moss taking hold. When moss spreads from spores or fragments it first throws out a downy film called protonema, threads that are genuinely green because they photosynthesise. A green haze is young moss getting going; a white or grey fuzz on the leaf litter is fungus about its own business.
What to do about each
Once you have named it, the response follows on its own. If it is moss on an outdoor surface, you can leave it where it does no harm, or lift it off if it makes paving slippery or a roof untidy, which the guide on removing moss works through. If it is mould, cleaning the visible patch is only half the job, since unless the damp behind it is dealt with the growth simply comes back, so ventilation and the source of the moisture come first. Two neighbours often cloud the picture. A flat green film with no leaves at all is likelier to be algae than either moss or mould, a difference drawn out under moss versus algae. And a moss you were sure had died, gone brown and crisp, may be perfectly alive and merely thirsty, a false alarm untangled in why is my moss turning brown.