They live off the algae on the rocks no prob.. had a potter's angel that didn't eat offered foods for quite a while... he was just doing what they do naturally and eating algae..
In the future, one could also take algae sheet and tie it to a chunk of live rock with fishing line to somewhat simulate natural growth..
It helps the fish get used to the taste of the food while presenting it in a less alien fashion then a clip on a glass wall with a human staring at it
Glad he is eating, be sure to get it on something besides the pellets tho, he'll need a well rounded diet (of sorts).
Garlic is a great thing, Selcon is a great thing, Zoe is a great thing. Getting that angel to eat something soaked in 2 of the 3 (I've never mixed selcon and zoe at the same time, afraid it'd be overkill) would be an awesome thing that'd bring out it's color and keep it happy and healthy.
(my now-not-shy potters angel that will eat from my hand and is strikingly gorgeous is a testament I'd think)...gl as always and ENJOY..