When you get them eating they tend to like have a voracious appetite. I would use frozen foods, clams, and ulva, and spiraling macro algae. For harder angels sponges growing in a cryptic zone would be ideal. Many approaches would work it just depends on the money and time you want to put in.
In the wild most angels like sponges and algea. In home aquarium they can get accustomed to any kind of food. It is better to feed angel a verity of foods. I feed my angels every thing pellets flakes and frozen food mix that contains meaty stuff like fish, scallops, shrimp, mysis, clams, oysters and nori.
You can also check my food article for more details