I've had my 3 sun colonies for a while and for months I fed nothing but pellets... I think in nature, they survive mostly on marine snow, so whatever falls on them. So pellets shouldn't be too hard to digest, it is in water. :)

suncoral
 

i feed my 2 yellow and orange suncorals only mysis. they must be contented since i'm finding couple small sun polyps dotted about the tank.

Seems to be unanimous. They'll eat anything that falls on them. Feed them a variety of things and they should do well.