Thats definately a Stone Crab, I had one just like it hitch hike. I wish it was the good kind of stone crab, the ones sold in Miami for a lot of money, but alas, hes a nuisance. Kill him off or throw him in the ocean!
Technically speaking there are no "good" crabs they are all oportunistic feeders. Personally I don't mind crabs too much although I never had a bad experience with them. But then again I am one of those reefers who actually bought a mantis shrimp! LOL
put him into your sump..he dosent deserve to die because he dosent fit with your other tank inhabitants...its not the crabs fault his home was taken out of the ocean and put in your tank..so please put him in your sump or put him back in the ocean
I'm with boomsticks; I've got several crabs in the rock in my tank (both "good" and "bad" types), and until they get quite large they never give me any trouble. When they turn into a problem I remove them on a case by case basis, but until then I find they are just one more interesting thing to look at.
If that means I have to buy a few extra snails or hermits from the LFS a few times a year, no biggie.