Identify this crab, PLEASE!!!

I turned my lights on and found this guy crawling around.  I was a Mountain Shadow one day when one of the guys brought one just like it in.  Randy mentioned they were bad for reef tanks.  Any one know if this is true?  What kind of crab is it?

Did you hear them say the term "Rock Crab"?
Rock crabs typically come in on uncured LR and as they grow larger their hunger causes them to eat anything and everything. Definitely coral and fish that they can catch.
There is an easy way to catch them. Simply place a piece of raw shrimp from the grocery store in a glass jar leaned up at about a 60 degree angle against the rock they may be living in or under. When they slide down into the glass to eat the shrimp they cannot crawl back out. They usually go out at night. You can often find their hiding place by looking around with a dim flashlight at night.