My husband and I are fairly new to the marine scene as well, but had the same problem with a hitchhiker crab.
We did a lot of research on if it was good or bad, and with help from a lot of people on this site (gotta love that!), we found out that our hitchhiker crab, was indeed, very bad.
This site can possibly help you ID him
http://www.flmnh.ufl.edu/reefs/guami...dae/index.html
We were told to prop a glass up against the rock, and place a shrimp (cocktail, or raw) in the bottom of it. Do this when all of the lights are out, and the crab should fall in to get the shrimp and not be able to get back out.
As well, the owner of a LFS (Ocean City) here told us to place a shrimp in the middle of the tank, turn the lights out, and when the crab comes out, to shine a flashlight on it (which supposedly immobilizes them), making it easy to fish it out.
Hopefully that will help, and then you can put them in your sump, or do with them as you wish.