I would say nthers will chime in.Doesn't look like your typical gorilla crab or the stone crab.Here is something I found at a link.Looks like your culprit.
That's why when you get live rock and cure it,dig in all the holes and crevices with a screwdriver and dig out whatever you can,sometimes they just show up anyway
keep him in your sump. he may not look pretty and not good in the display... but he'll probably eat up whatever crap that gets down into your sump and he will keep it clean.
Def a Gorilla..I had some kind of evil lil crab that came on a colony I bought. I tried to be nice and put him in the 1st chamber of my sump..one day I look and he's not there the lil fawker climbed up my filter sock and went into my main chamber going on a rampage killing a ton of astrea snails. Def flush him it's not worth it to save him compared to how much other CUC you could lose. I gave mine a chance because I was unsure what kind of crab he was but yours is text book gorilla and they are nasty.
True but damn kill him????? Ahhh does any one want him he's kinda cool and scary looking atvthe same time all black with red eyes.... Pick up in queens good for a all fish tank....
leave him be... Unless you really feel like he is going to do harm to your tank. I have my overflows covered with mesh so anything like that wouldn't make its way up the drain.
There are people who want crabs for octopus food.
Or he could go in a FOWLR. Post him as a freeby. I wouldn't house it long term in your sump. He could cause problems in the long run.