Are all hitchhiker crabs bad?

[cfgenesis]

My live rock has been in my tank for a couple of weeks and last night I noticed a tiny brownish crab sitting at the entrance of a hole in the rock feeding himself off the nearby particles. It is definitely not a mantis, and doesn't look hairy like a gorilla crab. Should I just assume it is bad news and terminate it??

[DRTutoki]

A picture would help. Hard to ID with description.

[sage_commander]

Not all crabs are bad. A porcelain crab is peaceful and feeds by waving "nets" in the water - they're filter feeders. They do have claws for defense, though.

[Powdermonkey]

"Fuzzy crabs, crabs with blue eyes and black pinchers, and huge crabs with red eyes are the ones we normally encounter and try to keep out of our tanks, since they tend to be predatory to smaller fish and opportunisitically may devour other things we care about."

[swigen]

+1 on the porcelain crab. I got a tiny (under an inch) spotted one that came in on my live rock. At the time I was worried so I caught it and easily IDd it as a porcelain. I dont see it much but its completely harmless