Looks like a mithrax to me, but can't be absolutely sure until I see the cups on the end of the pincers. I have at least two good sized mithrax in my 77 and they've been great scavengers... without bothering any of my other 'desireable' critters. I have no plans on catching them until I've seen some damage and there hasn't been any since I bought my system used. I assume they were hitchhikers as well. I've seen them scrape algae, coralline off the rocks and glass. Generally quite wary and are spooked fairly easily by anything approaching them.

I saw a crab the other night in my system, my tank is only about 4 months old and i bought all brand new live rock from oc and i did a soda water dip on it all and about 4 crabs jumped out, but i lost one of the chromis in my school im not saying the crabs the one that did it, but I can only assume.