Algae or what?

[Phoenix7506]

Ok so I've had this stuff growing for a few weeks on one particular rock. I just thought it was a mat of algae and would go away with time as the tank is fairly new. I haven't really worried about this stuff and I'm still not but I was moving a piece of coral and thought to myself, I wonder if I can peel this algae off the rock. When I pulled it off it came off in a ball shape and was about a half inch thick. On the underside it is white and it is firm and rubbery. Here's a picture. I'm curious as to what the heck this is. Never seen anything like it.

[Steve]

Not for sure but it looks like some type of sponge to me.

[Phoenix7506]

I was kinda wondering if they were. The underside of my rocks are covered in pineapple sponges. I hope someone knows what exactly it is. I'd like to know if I need to take it out.