causes of death for an emerald crab?

so just now i just checked on my tank as i do fairly often through out the day and i notice my emerald crab laying on his back on the sand bed...so my first reaction was that he had molted and pulled the shell out only to find no exit on the shell which made me realize he hadnt molted and was dead, he was 100% limp and as you moved it the legs and claws just slid from side to side with your hand i know he wasnt laying there earlier and have seen him active as recent as of yesterday scurrying around my LR

there was no sign of injury and all other fish/corals/inverts seem to be healthy and fine tank has never been treated with copper and i dont dose iodine other than the iodine that comes with my water changes

here are some pictures





what could have caused this?

 

how long have you had this little guy :/ is there lots for him to eat? when there little they eat algae as they grow older they eat more meat foods and poop!

 

over a year and yes he was well fed, if anything i could feed less sparingly