Brown moss sets off a small panic, and it usually should not. Moss turns brown for several quite separate reasons, and only some of them mean the plant has died. Before you bin a patch or drown it in a can of water, it pays to work out which brown you are looking at, because the remedy for one cause makes another worse.
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Usually it is only thirsty
A moss has no waterproof skin and no roots feeding it from a reserve below. It drinks over its whole surface and simply dries out when the air does, so a spell of dry weather turns most mosses a dull olive, straw or greyish brown within a day or two, the shoots shrinking and curling as the water leaves them. Alarming as that looks, it is very often nothing of the sort. The test takes a minute. Wet a pinch of the brown moss, or stand the patch somewhere humid, then look again half an hour later; if the shoots swell and green up, the plant was merely dormant and has now had a drink. Bringing a badly shrivelled patch back is covered under reviving dried moss, while watering moss deals with stopping it drying to that point at all.
Scorched by too much sun
Where a shade-loving moss is bleaching to pale straw or white at the tips, and worst on the side that faces the light, the trouble is sun rather than thirst, and a watering can will not mend it. Woodland and terrarium mosses scald especially fast, needing only a few hours of direct summer sun behind glass or on a bright sill to burn. The bleached tips seldom recover, but the shoots below them usually do once you shift the moss into steady shade or bright indirect light and let fresh growth hide the damage. When this happens inside a jar it tends to come with the other glasshouse faults gathered under terrarium troubleshooting.
Drowned, and starting to rot
Brown that arrives with sliminess, a sour smell or tufts of grey fuzz is the opposite fault. Moss left standing in water with no air moving over it suffocates and rots, and the rot creeps outward as a dark, mushy margin rather than an even fade. This is the classic mistake in a sealed terrarium that is misted daily and never opened. Ease right off the water, lift the lid to let a draught pass through, snip away any blackened or mouldy tissue before it travels, and sort out the drainage underneath. Living moss wants steady damp rather than a bog around its base, and getting that line right is most of the battle indoors.
Hard water, salt and feed
Now and then the water itself is the culprit. Because moss absorbs whatever you spray on it straight through its leaves, it is fussy about what lands there. Hard tap water leaves a chalky bloom that can slowly brown a sensitive species; artificially softened water is worse still, since it trades the chalk for sodium, which moss cannot abide. A stray splash of lawn feed or ordinary houseplant fertiliser will burn it outright, browning a patch inside a few days. Rainwater is the safe default for all of it. If you have been feeding moss in the hope of perking up its colour, that is very likely your answer, and the reasons it backfires sit under feeding moss.
The browning that is meant to happen
Not every brown is a wound. Plenty of mosses shift colour with the seasons, some bronzing or reddening through a cold, bright winter and turning green again in spring, so a patch that looks scorched in February may just be biding its time; those swings are the subject of moss through the seasons. The base of an old cushion browns quite normally too, the living green tips riding on a thatch of their own spent lower growth. And the fine brown threads that rise from a healthy green patch in spring, each capped with a tiny swelling, are not decay; they are the moss in fruit, its spore capsules, and a sign the colony is thriving.
When brown moss is simply finished
One last case trips people up. Preserved moss, the soft dyed sort sold for framed walls and displays, is not alive and cannot be revived. If it browns, dulls or goes crisp, it is reacting to sunlight or to damp air, and no watering will help; water in fact ruins it. Keep it clear of direct sun and out of humid rooms, and read any browning as a sign it has reached the end of its display life rather than a problem to fix. Why preserved and living moss behave so differently is something I set out under a preserved moss wall.