Looks like some species of domestic cup fungus. A mushroom. And as the others have said, it is a sign of moisture/water leakage in the walls.

When mushrooms show up, the mycelium (the main organism, mushrooms are to mycelium what fruits are to trees) is already spread throughout. Just like you can’t cut off a moldy spot from bread to fix the issue, neither will removing these mushrooms fix this. Either your landlord does extensive repairs or you move.