Dont worry, they arent like normal mushrooms on the land haha. Those are corals, so they wont harm anything. They do tend to spread really quickly. They didnt just show up overnight, either the rock is new, or someone added the mushrooms, or you just really really havent seen them, which would be hard to ignore them hahaha.
From the pictures, Im going to take a wild hit at the type of them, but Im not very good at IDing them. Id say the red ones are of the red spotted variety. SWF.com only has a few different kinds that are spotted, but you may notice the similarity.