Picture of MOLTING
When fiddler crabs molt they will eat very little and will stay upside down for many hours, DO NOT touch or mess with your fiddler crab when its molting! Fiddler crabs may look hard but are acutally VERY FRAGILE!
I once accidentally KILLED one of my females when I tried to help it back up...
After your fiddler crab is done molting you may see him eating the old shell THATS OK,fiddler crabs will eat the old shell for nutrition,DONT TAKE OUT THE SHELL EVEN IF THERE ARE NO CRABS AROUND IT! Your crab needs the nutrition and will only it the shell when no ones around (exception for other crabs)
Be sure to isolate your fiddler crab when hes done molting,after he starts to be active and his new shell comes in you can put him back in the main tank...