We have or maybe had (haven't seen him in a few months) one in our tank. He never hurt or went after anyone or any corals. All we saw him do was eat algae and detritus off the LR at night. Our sexy shrimp would hang out with him at night and they got along just fine. He was about 1"-1 1/2" last I saw.

If I ever see a crab in my tank, it's 455-whoopin' time.

I actually saw a hairy crab attack and kill one of my cleaner shrimps ninja-style. Shrimp was mellowing under an overhang when the crab came down and snagged it.

I swear to god you cannot make this stuff up. I was livid.