Hmm...I would say pistol shrimp. Is this new rock from the same guys? Take a small coke bottle, cut off the top, flip it upside down and stick it back in. Cut a small hole and stick a twist tie or something to make sure the top doesn't come out. Then put some food inside of the little bottle and put it back into your tank for a few hours periodically checking back to it.
Build a Better Fish Trap