my personal opinion-dying. crabs usually try to stay hidden when they moult because they are very vulnerable to predators when they are all soft and squishy. and from my own experience with emeralds, they dont like to be out in the open that much specially in the light. but you never know, he might not know that. if he is still alive then leave him be and see what happens. he looks pretty big though. i think you might find he starts eating the wrong stuff pretty soon and you will want to remove him. you might get lucky though