Trying to I.D.?

[Gillfuy]

This purple thing has been growing on this rock for the last six months. It has the shape of a mushroom but it more like a fungus because it's completely attached to the rock. There is no underside to it. It started as a small dot and now it's about two and half inches wide. Can anyone please I.D.?

 

[Scooterman]

I can't from the pic but good thing it is growing slowly, If you do decide to remove it, I would do it in a bucket, then rinse well & not put any of that water back in the main tank.