Looks to me like a one of the Leucosiidae crabs (aka pebble crabs), by the shape of her shell... It's not concidered a "reef safe" crab though with caution would be fine not in a reef or with small fish/inverts. As the only truely reef safe crabs are the commenisle coral crabs and SOME porciline crabs.
yeah looks like you got the id correct, hes decent size 2 in... dont see anything on people keeping or selling these in the trade, anyone have any info on temperment, diet ect?
They are likely omivours, probably leaning toward carniverous as they get bigger. Sounds like you have a pred tank or fish only? In which case you probably wont have any problems unless he gets big... (not sure the max size on these guys).
He will more than likely get snacked sooner or later with a stock list like that. Just wait for him to molt one day when they find him...He cant hide forever.
No what makes me nervous about it is that it dosent attempt to hide, just digs in to the sand hides legs and claws and sits .....dosent move for 18hrs.... Waiting watching...but he's easy to catch