I think it's a Mithrax crab. I have both a Mithrax & Xanthid in my system. I believe a Xanthid has somewhat pointier pincers than a Mithrax. Over the years I've observed two larger & a couple of smaller Mithrax in my tank. From what I can tell they haven't harmed any of my healthy fish or other livestock. When my BTA was fairly new & sickly, it would try to take scraps of food I was trying to feed, but that was years ago. Healthy nem now & the crab stays away. One of the larger Mithrax appears to have been killed off, probably battle between the two larger ones, since each was missing some body parts a while back.
The Xanthid on the other hand, I think is responsible for chewing up a Ricordia frag I had just placed on the rockwork last week. Less inclined to think these guys are a good thing to have in a reef than a Mithrax. In the end, either could probably do some damage, but for the most part, Mithrax is beneficial as an algae, coralline grazer & food scrap cleaner upper.
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