Same thing happened to me the first time I got an emerald crab. When I came home from work the day after I'd gotten him i saw him lying upside down on the sand all white. I was so bummed. While i was pulling him out, i was startled as he scurried from under a rock. Thankfully, I was just pulling out his molt! Looked so much like a whole crab. I asked the lfs about it and was told that they often molt when there are chemistry changes to the water, but they also molt regularly to grow. I had him for a long while and he molted probably once a month.