Looks like a member of the Galatheid Crab family, AKA squat lobster...
Don't know specific species or whether it is harmless. Maybe Donya can chime in...
Looks more like Inachidae to me, the family that has arrow crabs (the common Stenorhynchus species in the trade). Unfortunately it's a family I'm not familiar with in detail. This one might be Pleistacantha oryx?