you can rubber band the crab's shell to a rock and stick it in a plastic container with tank water, and put an empty shell that's about the same size in there. the crab will eventually leave the shell with worms and go into the empty shell and you can put it back in your tank.
It kind of looks like his shell is covered in Colonial hydroids... if that what those are they are bad and you'll want to get the crab into a new shell and chuck the covered one in the trash before they potentially spread to your rock work. Assuming thats what they are, I just can't think of anything else they could be.