The marimo is the friendliest oddity in the aquarium trade: a soft green velvet ball that asks almost nothing and can outlive the keeper. The first thing to know is that it is not a moss, despite the name stuck on it everywhere.
What a marimo actually is
A marimo is a green alga, Aegagropila linnaei, that in a few cold, clean lakes in Japan, Iceland and northern Europe grows into dense spheres rolled smooth by gentle currents. The ball is not a single organism wrapped round a core but a colony of filaments growing radially, green all the way through. Wild marimo are slow-growing and rare enough to be legally protected in their native lakes, so the ones sold are cultivated or hand-rolled from loose filaments rather than taken from the wild.
How to keep one
Marimo want the conditions of the cold lakes they come from: cool water, gentle or no flow, and modest light. They are happiest below about 22 degrees and dislike a hot, brightly lit tank, which is one of the few ways to actually harm them. Keep them in clean, cool water, give them an occasional swill in old tank water or dechlorinated water to rinse out trapped debris, and that is more or less the whole regime. Looked after, they live for decades; some kept specimens are over a century old.
Keeping it round and green
Two simple habits keep a marimo looking right. Roll it gently in your hand now and again, or rely on a little water movement to turn it, so every side gets light and it stays spherical rather than flattening where it rests. And keep it out of strong light and warmth, which bleach it pale or let it brown and fur with other algae. If a ball goes patchy, a rinse, a gentle squeeze and a spell somewhere cool and dim usually brings it back; a badly misshapen one can be rolled back into form by hand.
In the tank, and with shrimp
Marimo sit happily in a community tank and are a particular favourite in shrimp tanks, where the residents graze the biofilm off the surface and pick through the filaments for scraps. They are soft, harmless and trap no sharp edges. Because they are an alga rather than a rooted plant, you simply set them on the substrate or wedge them where you like. For the wider untangling of which aquarium "mosses" are the real thing, see which aquarium mosses are really moss.