People bring moss indoors expecting a low-effort green and watch it turn brown in a fortnight. It is not that moss is fussy; it is that a normal room is the wrong climate for it. Set it up in the right way, though, and moss is one of the most trouble-free green things you can keep inside.
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Why it browns on the windowsill
A heated room is dry, and dry air is moss's main enemy indoors. Moss has no roots and no waxy skin to hold water; it drinks through its leaves and loses moisture just as freely through them. Out in the open air of a living room it loses water faster than it can take it up, dries out and goes dormant, and if it stays dry for long it eventually dies rather than merely resting. Direct sun through glass finishes it off faster still, scorching a cushion to straw in an afternoon. Open moss in a centrally heated room is fighting a losing battle from the start.
The reliable answer: a closed terrarium
A lidded glass jar or bowl traps humidity and recycles it, which is exactly the climate moss evolved for, and turns it into a near-zero-maintenance feature. Water goes in once, condenses on the glass, runs back down and is used again, so a well-sealed terrarium can go weeks or even months between waterings. This is the honest answer to keeping moss as a houseplant: grow it behind glass and it will thrive for years. See the terrarium guide for how to build one, and the species roundup for the mosses that hold their looks under a lid.
Open dishes and bonsai
If you want moss in the open, on a bonsai or in a shallow dish, you are signing up to mist it daily and keep it out of direct sun and away from radiators. It can be done in a naturally humid room such as a bright bathroom or a cool north-facing kitchen, but it is hands-on, and a week away with nobody to mist it will set you back. A room sitting at higher humidity, well above half, makes it far easier; a dry lounge in the depths of winter makes it nearly impossible.
Which mosses cope best inside
Some mosses tolerate indoor life better than others. The cushion-forming kinds such as bun moss hold their shape and colour under glass and are the classic terrarium choice. The flat, carpeting sheet mosses are more forgiving in an open dish because they knit down and recover well from a short dry spell. Wild-collected moss from a shady bank often settles more happily than anything bought dried, since it is already used to low light. Whatever you use, give it a gentle rinse and a look-over for pests before it comes indoors.
Light
Moss does not want strong light, but it does need some. Bright, indirect light is ideal; deep gloom leaves it weak, thin and prone to algae taking over the surface. A spot near a north window, or a low-output grow light for a closed setup, keeps it healthy without cooking it. If a terrarium starts growing more algae than moss, it is getting too much light, and simply moving it back from the window usually settles things down again.
Every indoor moss failure I have had came down to dry, still air rather than light, and it took me a while to accept that a warm centrally heated room is a hostile place for open moss. The moss on a saucer that I misted daily still crisped up, because the air around it dried between sprays; the identical moss in a sealed jar beside it thrived on being ignored. So my honest advice for indoors is to stop fighting the room and put the moss under glass. If you want open moss on display, mist it far more often than feels reasonable and keep it well away from any radiator, and even then expect to baby it.