Crabs!? ID please.

[doch]

So I have no pics due to no camera taht would work, but even if I did... wouldn't help much cuz he's FAST! My desription may not help anyone but it's wortha shot: Small crab, about 1/2" - 3/4" side to side, looked te be grayish in colour with what looked like a burgandy stripe in the front, posssibly stretching to the back. This is the first time I've seen him, and well, I'm kinda curious if he's doing any harm to anything. I haven't noticed anything, but who knows how long he's been there? Anyways, any guesses are appreciated!