that is strange, i have 2 in with my seahorses and you'd think they would be easy enough to pick on but they get along just fine, was planning on adding 2 more tomorrow but perhaps you could remove the crab to the sump/other tank/my tank *nudge nudge* and see if the situation gets better

This seems odd, I have a BS jawfish and at least 10 boxers in with him and never had any problems. Wrong place at the wrong time ? No other fish or such that could be stressing him out ? I had a lawnmower blenny in with him once and it turned aggressive after a couple of months together and had to take it out.