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Chase him and when he gets inside a piece of live rock take it out of the tank and you can drizzle carbonated water (selzer water) and he will come out, it will kill a minimal amount of living things so you will still have some. If it doesnt work do a peroxide dip.
It's not a big deal. Just a rock boring xanthid crab. Very common. A crab trap would be a good way to remove him. I have a few of them which were banished to the sump. Just as destructive as emerald crabs IME.
Try to make a trap out of a small glass like a shot glass etc. lean it up against the rock. Put some bait like frozen food or pellets. You may have to put the food inside a media bag or small piece of panty hose so it stays in the glass and not float away. Set it up so when the crab goes in for the food he can not climb out. And just sit back and wait. Might have to turn the lights off so it comes out.