I too have the sun coral and it was doing great when I fed it faithfully (twice a week) but now it is showing part of it's skeleton (as yours is.) I'm afraid to tell you that unless you can reverse this trend, you will not have this coral for very long. Most of the literature out there tells you that they like to eat after lights out but mine would extend it's tenacles during and day and readily take food (if the fish didn't get them first.)
The light is not an issue, the viligant feeding is (and yes, Alyssia is right - all of the stems are separate and must be fed individually.)
I hope this helps.