Mystery Crab Hitchhiker

[i am batman]

I came across a carcass (or possibly a moulted body) of a crab floating around in my tank. I purchased my live rock from a fellow reefer and it's possible that the crab came from there. My only concern is that I cannot find any crabs with the

[hr]
eye as I watch this tank religiously and have had no luck finding them. I'm sorta new to this whole reefkeeping game and don't know if I should be concerned enough to start turning over rocks before one unseen starts eating my snails/inverts/fish etc.... The crab shell appears to be pretty large and doesn't appear to be a typical reef dweller, but what do I know? So in other words.......
Should I be concerned? Thanks