Ummm
They look more like what I call button anenomes. They will overtake the whole tank. They are like aptasia, but they don't sting coral as bad.
They are not a good thing, kill them like aptasia.
But if you think they are mushrooms, wait... If they develope a stem they are the things I am talking about.
I counted six already on the rock. 3 lg and 3 tiny ones