Xanthid Crabs are very destructive crabs. While they are small they tend to feed mostly on algae. However, once they get bigger they will start to feed off other invertebrates and will even start feeding on corals (mostly acropora). They also have a nasty little habit of expanding their den (usually inside of your live rock) which will weaken the rock, and in some cases will cause a collapse.

There are several hundred species in the family Xanthidae, which can make identifying this particular crab to the species level rather difficult. However, black-tipped claws are a good indicator that this crab was bad news (not all black-tipped crabs are bad though)

A good trick to catching crabs is to lean a drinking glass up against a rock (close to where they hang out is best). Stick some food in the bottom of it, and put it in the tank right when the lights go off. If the crabs go after the food, they won't be able to climb up the glass to escape, so in the morning you can just take them out of the tank.

Sources
http://www.chucksaddiction.com/hitchcrabs.html
http://www.reefdup.com/2012/07/06/invertebrates-crustaceans-crabs-part-ii/