That's a pineapple sponge, they volunteer in new tanks quite often. They don't mean your tank is ready to host all types of sponges though.
I read this on another forum about their specific identity.
The term "pineapple sponge" is much over mis used. By simply looking at these things, there is no way to know what it is. Though, it may be safe to say, that it is most likely a calcareous sponge, and harmless. It takes microscopic examination of the spicules themselves, to make any sort of identification. These things could be from any one of a number of different Genera, other than Scypha, within the major group Calcarea. The Genus and species cannot be pinned down by looking at a picture. Even sponge experts won't try, without a compound microscope.