Ghost shrimp eat a lot of biofilm. They will do fine left without being fed anything for at least a couple of weeks, and that's safer than using an auto feeder when nobody is around to check on it. More shrimp die of poor water than ever do from starving awhile.
If they have moss balls, they'll be fat and happy for a long, long time without additional feeding.
EDit. They don't eat fish poop. They eat dead plant matter, some algaes, biofilm, fungi, dead fish, dead shrimp, dead anything else as well, but not fish poop. Not much of anything eats fish poop, though I suppose some snails may eat a bit of it now and then.