The purpose of the clean up crew is to help consume any uneaten food and break down uneaten food and waste even further. some will help in consuming nuisance algae.
Here are a few examples of what you can introduce.
Hermit crabs. These are the first things many reefers add to their aquarium, most common are red legged and blue legged crabs. Don't forget to provide a few extra shells as they like to change from time to time. If no empty shells are available they may fight for them.
Snails. Turbo snails are the most common introduced to marine aquarium. These will help in the process of cleaning your glass and live rock from algae, they will not perform miracles though, you still need to clean the glass frequently.
Shrimp. I would start with a cleaner shrimp as these are less expensive, one or two for most aquariums. Peppermint shrimp come highly recommended these will eat the pest anemone, aiptasia. The favorite of many reefers have to be fire shrimp or blood shrimp as it is otherwise known. a fantastic bright red shrimp and white legs, stunning but expensive.
Starfish. There are many varieties of starfish available and some are very difficult to keep in the home aquarium. You should research then decide if suitable. It wont do any harm in introducing sand sifting starfish as these are really helpful at cleaning the sand.
As with any fish are coral these are sensitive animals and need to be carefully and correctly acclimatised particularly starfish.