the idea of a "cleanup crew" is that they will clean up the detritus and any nuisance algae that is left by the cycle.
the truth is, you don't need a cleanup crew. this was created by livestock vendors to make sales.
if you "believe the hype", then the idea is that you don't have to feed them anything.
my suggestion would be that if you like crabs, shrimp, snails and such and want them in your system, add them sparingly. do not go by the ridiculously inflated "10 snails and 20 crabs per gallon" nonsense you see on these CUC websites.
to answer your question-
snails will eat specific algae
crabs will eat snails and whatever else is available
sand sifting stars will eat very little, then die of starvation