I've heard of and seen sponges that crowd out or even smother some corals, but never heard of one that has a stinging capability. There's tons of things that can come in as hitchhikers on rock, so a definitive answer may be hard to come by, especially without a clearer picture. Just from my limited knowledge of sponges, I'm having a hard time accepting this as a sponge. They are basically very simple organisms that are built to push water through their system. But like I said, who knows what's down there?