Why Do I Have A Mushroom Growing Out Of My Bathroom Ceiling?

There must be a leak. Behind the wall or ceiling there is probably a leak which has caused mould to grow. From the mould, fungus grows. You need to get this seen to straight away. The fungi can feed on rotting wood, so any wood will need to be taken out and replaced. The leak will need to be fixed first, and then the surrounding areas should be kept dry.

Dude girlfriend has the same problem in her salon...

Mushrooms grow from spores, not seeds. A blend of the spores and these nutrients are called spawn. The spawn supports the growth of mushrooms tiny, white, a thread like roots, called mycelium. The spawn itself could grow mushrooms, but you'll get a lot better mushroom harvest when the spawn is applied to a substrate or growing medium. Mushrooms prefer dark, cool, moist, and humid growing environments. Most mushrooms grow best in temperatures between 55 and 60 degrees F, away from drying, direct heat and drafts. So if the spawn got perfect environment then that can grow in any place.  Agrinoon produces the quality based fresh and dried mushroom in the market at the best prices. Thanks.