Any crab is an opportunistic feeder and should be regarded as suspect until proven otherwise. If it is a Mithrax and has the blunted claws as Jason describes then treat as you would any Emerald(Mithrax) crab.
That said I have two smaller red eyed, pointy black clawed crabs(Xanthid) that I'm sure does some damage but I leave in my tank as part of the CUC and I think they look neat.
Crabs are a personal thing.
