Detritus may be the best food for grass shrimp however they need the proper cover habitat with detritus to survive fish predation. I know nothing about the specific calcium content of the water for their survival. If you can get crayfish to thrive, the grass shrimp with the same chitin like 'shell' will also thrive. Again, it is the type of habitat requirement that makes for a successful grass shrimp population. Habitat, Habitat, Habitat. The best habitat IMO is finely divided leafed submerged vegetation. They can survive without ample proper vegetation, however the protection from predation pressure needs to be conducive to all their life stages. Remember - their life cycle is one year.

When the pond has lots of dense habitat this means there is an abundance of natural foods for grass shrimp. Looking on YouTube, aquarium type people feed the grass shrimp algae both filamentous(FA) and attached algae(periphyton). fish food both flake and pellet. Some say they even eat fish manure. They probably eat most anything that is organic based thus the feeding term for them is detritivore. About every pond will have plenty of food for them. It is the good type of habitat that is important for their survival until they can reproduce and not get eaten.