Fiddler Crab problem?

I went downstairs and checked on my crab, he was upside down. I picked him up and he moved very slowly, normally hes running at top speed to get away from me . My Fiddler crab has difficulty moving. When I put him back in, as he moved down the tank he flipped upside down. He can right himself a little now. What can i do? I tried feeding him, he didnt even try it. I read you have to leave the shell in there so he can eat it after he moults. I picked the moult up and forgot to put it back in. I heard they need the calcium they lost while moulting. If the problem is i forgot to leave the moult in, what can i feed him so he can restore the calcium? He seems to move his mouth parts at a nowmal rate so maybe he can eat.If a low calcium level isnt the problem, then what is?