Can't see from the pics. That's a staghorn crab.
Get a few empty shells in the tank just in case, and I hope your lighting is sufficient for his shell to grow. Since he'll always stay on the sand, furthest from the light, he might not do well for too long. Unlike others, since his shell is coral, it'll grow with him. They will take another shell if they can and their own shell doesn't get the lighting it needs.