unknown crab

I am new to the marine scene, and in a live rock purchase I "recieved" a crab that is chalk/offwhite/grey in color with an unusual design on its carapace-it looks like an upsidedown tulip in a flowerpot.

Does anyone have any idea if this is a fish/reef safe occupant or should I have some concern? It is only about the size of a dime right now, and I have not observed any 'felonious' actions in it's temperment but...

Any info would be greatly appreciated .

Thanks- I don't want to take any chances, but am still interested in identifying this crab. The carapace is offwhite, and the design is a flowerpot shape at the crabs head with a 'tulip' coming out of the 'flowerpot' toward the rear of the carapace.

Thanks again for your input.