If you just put something next to the tip of their arm, they should crawl over it. Rarely would you have to physically put the star on top of it, but you can (just keep that to a minimum though). Anything meaty will do, krill, squid, silversides, shrimp pellets and even algae wafers. FWIW, they will consume some algal films, but in the wild they eat more meat...whether they prey on it, or eat something that is already dead. They will do whatever is the easiest meal.
If the tank is full of larger, messier fish, then you may not need to feed the CC. However, I always prefer that this be done regardless.