The primarily will eat just excess food around the tank. I've heard stories of some ghost shrimp eating algae as well. But they don't eat fish waste if that's what you're referring to.
For the record, nothing eats waste. Its a common misconception becasue snails and other bottom feeders will frequently sift through waste looking for undigested food. Fish can only absorb the nutrients from so much food at a time but that doesn't stop them from eating. As a result, when overfed, they will pass undigested food in their feces. Shrimp and other cleaners willfind this food and eat it but if you don't know better, it appears they're eating poo.