Molting skins are GOOD for them! Try not to remove them with water changes. They eat them for calcium mostly. You don't necessarily have a dead one around, the molt means they have moved into a larger more comfortable skin lol Like a snake molt. They do this periodically, nothing to be freaked out about (*ahem* o-e)
Really, the biofilm grows on everything so there's very little chance of you "running out" of it lol shrimp don't need a whole lot of food so they scavenge all day long eating tiny tiny amounts.