generally pointy claws = bad
now take it fwiw, i have crab that was a hitchiker that the PO of my tank never new was there. It had to have been there for at least a few years as he hadnt added any new live rock or corals to the tank in that time. After i had the tank for a while i heard some popping and clicking from time to time. I thought great, a mantis. If a mantis had been in the tank that long he would have killed all the crabs and small stuff off long ago. So i started watching. One day i spotted out of the corner of my eye some movement where there shouldnt have been any. So i focused on where the movement had been and after watching for some time it turns out it was a little crab. He's been in there for a while, never comes out in full view. I've only been in a position where i could spear him once and that time i didnt get so lucky like with the big ol bristle worm. Only managed to knick a leg. Hopefully when it comes time to move tanks I'll be able to find him and dump him in the sump and relegate him to sump or fuge monkey status.
Probably going to do the same with my sally lightfoot crab when i move tanks. I've read that they can turn ugly when they get bigger. Mines pretty big already and its getting to that point to where i dont know if I want to trust it anymore. My banggai cardinal was all up in its face tonite making a threat display and the crab went about its business munching on little bits and bobs of algae and whatever else it could find on the rocks. It paid the fish no mind at all. On other occasions i've seen the crab take a swipe at my tang, but that is usually when the tang is being a pig and trying to eat everyones food, including the crabs.
Anyhow, Id get that crab out ASAP. If you see it hanging out in a specific area put a small glass on an angle against the rock work with a little bit of silverside or some other meaty morsel in it. Generally do this after lights out so the fish and other diurnal critters wont go after it. What will happen is the crab will come out, smell the food and make a b-line for it. It will go to the food and try to spirit it away someplace it feels safe to eat it. Well, since the glass is smooth the crab will have no traction and be stuck and waiting for you to pull it out. If you have a sump toss it in there, if not take it to the LFS and see if they want it (back, if thats where you got your rock from) and if they wont have it back toss him in the freezer and lights out for crabby.
