Hi Suzi, that's a cool little crab! What corals have you bought recently?
Sadly as @TheJag77
says, most are not considered reef safe.
I've had a look through the common hitchhikers
and can't find a match to the colouration.
The problem is most crabs are omnivore scavengers and have the potential to eat snails or corals. Not saying it definitely will do that, but there is a chance.
It's more likely to do this if the pincers are sharp, as opposed to the flat faced ones that mithrax crabs have (see attached pic)