[Quinn]

Remember that most crabs are opportunistic feeders and therefore if you have small, bottom-dwelling/sleeping fish (gobies and blennies in particular I've found), I wouldn't trust any crab with relatively large claws. And of course your corals could well be at risk*. According to Shimek a better indication of risk would be claw shape/colour, rather than eye colour. In fact I think eye colour is a poor indicator of anything in crabs based on what I've read, and is a myth that should probably be smothered in this hobby. As noted, the safest thing is undoubtedly relegating any hitchhiker crabs you find to your sump or a tank specifically set up for them. Like Bev said, most I've had seemed to stay around their little caves, eating algae (at any on time in my 150 gal I could find perhaps three little claws protruding from holes, happily ripping bits of algae off the rock around the opening of the hole). I had a 2"+ monster (Garfield) who seemed the same, I never noticed any damage to corals and he was within inches of my best Acropora sp. colony. I did have a goby who seemed to loose chunks of her tail fin on occasion but I'm not sure if this was a crab or just my female dog-esque female clownfish.

* Remember though that just because you have crabs in your tank, and you are losing snails, fish, corals, etc., doesn't mean the crabs are responsible. Correlation is not causation, and the average aquarium is a pretty poor experimental design, due to the myriad of third factors present.