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It is similar to my emerald crabs but it has a hairy body here are some pics best I could get cause he won't leave the rock about the size of a half dollar and there is also a smaller one that looks the same he is about the size of a dime
Don't know what kind but it looks like a bad boy judging from its claws. Try the jar method of trapping it. Baby jar, small pickle jar etc... Place it at a angle I front of its cave and place a piece of shrimp or scallop in there. If its hungry it should climb in and not be able to get back out. Cover with mesh. You might be able to get a better Id once you trap it.
If the op has a sump that could be a good place to keep it. I had an emeral crab that went rouge kille my pulsing Xenia and a very large head of my hammer coral! That guy was evicted right away.