OK... Grass shrimp are also called ghost shrimp. They are brackish water shrimp. They are detrivores and algae eaters, so they'll eat most things that they can find. You can find Ghost shrimp species in almost any type of aquarium set-up, from fresh to full salt. I don't know what species you have and what sort of parameters it tolerates, but you are best off trying to mimic the environment it came from.
Ghost shrimp are frequently used as live food in the aquarium industry. You can probably feed it decaying plants, live brine shrimp, small worms like bloodworms, or algae. A varied diet of some sort is your best bet to get it what it needs.