If there is nothing wavy coming out of it and it does not move and has no polyps, I would totally guess sponge. If any of those other things are visible, then I'd say possibly sponge with other stuff, too.
One of the coolest things (imho) about having a reef (or really having live rock at all) is all the "freebies" we end up with in our tanks - whether they are "baddies" or not!