Moss attracts more folklore than most plants. A few of the common claims, and what is actually true.
"Moss is killing my lawn"
It is the other way round. Moss moves in where grass has already weakened through shade, compaction or poor drainage, taking the space the grass has vacated rather than driving it out. Clear the moss and leave those conditions unchanged and it simply returns, which is why the moss and lawns guide spends its time on the conditions rather than the moss.
"Moss damages trees"
It does not. Moss on bark uses the trunk only as a perch and draws nothing from it. If anything its presence speaks well of the air, since moss favours clean, damp conditions. A mossy old tree is a healthy sight, not a warning.
"Moss only grows on the north side"
A half-truth worth understanding. Moss prefers the cooler, damper, shadier face of a tree or wall, and in the northern hemisphere that is most often the north side, so the old navigation lore holds up loosely. What the moss is actually tracking is shade and moisture, though, not a compass bearing. In a damp wood it grows merrily on every side, so treat it as a hint rather than a reliable direction-finder.
"You have to pressure-wash it off"
Please do not, least of all on a roof, where the jet strips the weatherproof surface from tiles and slates and costs you far more than the moss ever would. A dry brush does the job without the harm. The roof guide goes into why.
"Marimo balls are moss"
A common one in the aquarium hobby, and wrong: the marimo "moss ball" is a green alga that happens to grow into a velvety sphere. Confusingly, Java moss really is a moss, so the trade gets it both ways round. The full sorting is in which aquarium mosses are really moss.
"It has no roots, so it must feed off what it grows on"
No. The fine threads underneath are rhizoids, and their only job is to hold the plant in place. A moss feeds itself from rain, mist and airborne dust through its leaves, which is exactly why it can sit on bare rock, glass or a roof tile and take nothing from any of them.
"Moss is a fungus"
It is a plant, green and photosynthesising like any other, just an unusually simple one without roots, flowers or internal plumbing. The confusion may come from moss often keeping company with fungi and lichens in damp, shady places, but biologically they are far apart.
"Moss means a damp problem in the house"
Moss growing on an outside wall reflects shade and moisture on that wall, nothing more, and does not by itself indicate damp indoors. It does not bore into sound masonry or "spread" through a wall to the inside. Persistent damp inside has its own causes worth investigating, but a green north wall is not the diagnosis.
"You cannot grow moss on purpose"
You very much can, and people have for centuries; it just asks for the opposite of normal gardening. Give it shade, damp, poor firm ground and time, and it spreads readily from transplanted patches or a blended slurry. The whole of the growing guide is about doing it deliberately.