new creature in my tank (uninvited)

[Ryan Joel Smith]

So i went into my tv room last night and flicked the lights on (not tank lights) and noticed a quick movement on top of one of my rocks. What i believe was some sort of crab ran when there was any light at all in the tank so i could just barely see him in the crevas of a rock. I'm worried that this may be one of the crabs that are not good to be in a reef tank. Right now at blue world aquaria they have one of these crabs as a pet at the front desk and from what i remember it was a heavily amoured little critter that was white in color. What i saw in my tank didn't seem to be as bulky and had what i think was some black spotting........What do you think it could be and what kind of damage could it cause (worst case scenario). I really don't want to take my tank apart looking for him so i was thinking that if there were certain things a "bad" crab would do to my corals or fish i would monitor that and then take the appropriate action if he is in fact a threat to my reef....