Red claw crab Dead or molting or..idk please help

im having a problem in my 10 g tank that right now only has a red claw crab guppy female and fry, and 2 ghost shrimp i was looking at the crab and saw that a leg fell off. i thought he was just molting but then the next time i saw him his claw was missing and he was upside down pretty sure dead. what happend did he just failt to molt cuz of water quality or just ****ty luck theres no way anything else in that tank harmed him/her i was told to put a lil sea salt in just to help molting and i always use a low level of sea salt in that tank so i dont understand it certainly wasnt too much....ie im confused can any one help