I've had up to 4 in my display. A couple have since disappeared & I had to dispose of another several weeks ago. Caught him red handed with a piece of ricordia coral that I think he removed from the rock, frag plug & all. Pretty sure it was a Xanthidae, Actaeodes tomentosus. Had pointy piincers. The other known crab resident (Mithrax, Leptodius sp. Family Xanthidae) has scoops on his pincers & I've always only seen him/her scraping the live rock, never witnessed him damaging coral. He will take food scraps of any kind as well though. When my BTA was new & sickly, the crab would try to steal chunks of silverside I was trying to feed the nem.
As far as ID for yours, the first one looks like it may be an Emerald, so relatively useful & harmless in most tanks. Check to see if his pincers have scoops as opposed to being pointed. The scoops are better suited to scraping algae & coralline off the rockwork, so that's what they tend to do most of the time. Pointed pincers are better at tearing stuff up, like your small fish or coral. I've never seen one like in your second picture, so no clue on that one.
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