Looks like a common hairy crab or a brown hairy crab.
Bad.
however,
Don't always go with the "hairy crab = bad" rule.
Many emeralds have hairy legs. Some closer species to the emerald that are black/ brown marble in colour are hairy as well (name escapes me)
First thing you should look at is it's claws. Sharp and pointy tips on the claws means it tears away meat, little suction cup/pad looking things on the tip of the claws means it rips off algae. Suction cups= reef safe.
In my experience though, if the crabs is hungry, it may nip at small polyps on SPS, regardless of the species.