I'm not sure what it is but I can tell you its bad. It will eat small fish, snails, pretty much whatever it can get ahold of. When I first got my tank stuff would occasionally turn up missing. At the time I had an algae problem and had to take my rock out to clean it. My biggest rock had something rattling inside of it. After leaving it out for a while one of those crabs eventually came out. Just by the shape of his claws I knew he was a problem. After getting rid of him I never had any more problems. So I have no real evidence agaisnt them, but I my little "reef court" I had enought evidence for me. I gave him to the LFS to spare his life. Crabs that eat primarily algae have rounded or flat spots on the tips of their claws. Those guys like that with the sharp points are designed for killing and ripping flesh.