I have those in my tank. They are Cladocora or Ivory Tube corals. I'm betting that they came from the Gulf of Mexico probably near Florida. All of mine (I have a dozen or more of them) came with my live rock shipment from Tampa Bay Saltwater. They do fine and aren't hard to keep. One of them is, like yours, attached to a clam. They will get to be a really pretty rust color and will come out more often. Enjoy em! The clam is happy in our tank and we don't do anything special in feeding it.