You dont have to feed them anything. Thats what they are for to clean up algae, debris, and scraps. I do directly feed my cleaner shrimp once a week with prime reef from ocean nutrition.
I've never directly fed any of my clean-up crew.....except for my 2 peppermint shrimp....
I like to feed my lps corals some sliced scallop or shrimp now and then..when I do I first give a lil to each shrimp.....keeps them from strealing from the corals
They should get enough food, if some falls to the bottom of the tank. My shrimp used to run up the tank until they got their piece of formula one or what ever I was feeding the tank at the time.