I'd turn off the flow & spot feed those sun polyps. Try mysis or maybe even soaked Formula 2 pellets. As long as you can still see the mouth, they might take food. Feed them individually.
Don't just sprinkle Cyclopeeze around the area because they won't catch it, especially without the polyps fully extended.

If that doesn't work, some suggest pulling the frag up into a bowl of tank water once a day & giving it a food bath for 15 min or so. Try all sorts of food & see if anything works. (I wouldn't suggest disturbing the colony with this method unless spot feeding doesn't work)