Can't tell from the pic but if it has polyps I'm going to say its some sort of NPS coral (Look up Fuzzy Carrot Coral). If there are no polyps, plus the fact that it's in a bunch of Zoa's that I'm assuming are from the same region as the one's I have their probably some sort of sponge.
I read somewhere a while back that wild collected Zoa's can be difficult to keep simply because of hitchikers like that dying off from exposure to air/tank conditions and what not. But never had any problem with mine lots of sponges in them.