LIGHT! Well actually their main food source is zooxanthellae that live in the anemone grown by the light. The anemone "like corals" make their own food with the light you provide.
Of course an anemone will eat for extra energy like most corals also do. If you want to feed your anemone I suggest squirting mysis shrimp into it or even placing small pieces of scallop into it's center. Once gently placed there the anemone will use it's tentacles to pull the food into it's mouth.
I feed my bubble tip when I think of it but it's always getting pieces of food while I feed the fish anyways.