You should never dig up a crab as this is very stressful! Also, a crab shouldn't ever need to be ISO'd unless it is an emergency or a newly purchased crab. They will bury and molt, and the others will leave them alone. Crabs also have a hormone that they can release to postpone molting if need be. If Rex was preparing to molt in the main tank, digging him up may have caused him to put if off due to stress. Leave him in the main tank with the others and nature will take over!! Alot of us here believe in the "hands-off" approach to crabbing. That means that we don't handle them unless we have to. We just make their crabitat as close to nature as possible and let them live in happiness!!
But, I would definately leave them all in the main tank and just let nature do its thing!! Welcome to
HCRU by the way, and keep us updated on their conditions!!