There are some decorator crabs from Japan that are fine and virtually all of the commensal SPS crabs are perfectly fine (one of my favorite creatures in my tank - bought the coral colony because of the crab). Not all hairy crabs are bad and there are a bunch of "smooth" crabs that can be devastating, Leslie H is the inverts guru on Reef Central and if you can get pics, I would post an ID request on the invertebrates forum on Reef Central or ask Ron Shimek at Marine Depot forums.
I had a 2 to 3" black hairy crab in my scorpionfish tank for two years and never bothered anything (including the Hawaiian frogfish that gets about 2"). Crab taxonomy is very weak and knowing the habits of many is very hard. On reefcentral the suggestion is to usually place in fuge which is absolutely ludicrous, you don't want any predators in your fuge!
Lee