What is this??

[Pier Pressure]

So these pinapple sponges - do they have little round indentations all over them, too, kind of like craters in the surface of the moon?

I lost a really nice colt coral, and this big round thing was at the bottom of it. I thought it was just a piece of the colt, but now an exact, smaller replica has appeared several inches away from the first. From your picture I cannot tell if it has the craters, though.