It should take both foods you mention without problem. I've even seen them take whole mysis shrimp before, but those really are too large for them. Small foods are best. Even filter feeding foods are good because they'll capture it on the mucous on their tentacles and still get it that way. They are mainly photosynthetic, but feeding a variety of foods a couple times a week will certainly help keep it healthy and happy.
That is a nice one, you don't see them that often with the whole bowl.....I have one that is about 12-14'' across, I have never fed it directly.....In the five years I have had it, it has grown about 3-4x the original size.....You should have good luck with it, I have found that a good amount of intermittent flow ( med/ high) really helps the polyps come out.....The polyps one mine extend about an 1 1/2'' with that kinda flow