It is somewhat flexible but will snap off if bent to far. That's why I am really thinking it is some sort of calcified macro algae. Non of my inverts mess with it, not even the emeralds. I do have several small frags. If anyone is interested in a trade pm me. I have no problem shipping it if you cover the cost. If it is a macro, it can be sent 3 day. Evidently it is a tough little bugger. When I fostered the rock from him, the tank was in complete disarray. Algae everywhere. Since then, he has gotten all his rock back and I have that "red? Coral" and the bubbles left. I do have frags without the bubbles and they are dyeing off anyway.