Sun corals are non-photosynthetic so it does not matter if they are under light or not.
I have mine right out under strong lighting...
To get them open... you have to introduce some form of stimulant eg. strong-smelling food into the water. My sun corals will open up once they detect my hands in the water (not joking), or when I feed GPs, cyclop-eze, brineshrimp, mysis etc.
You have to target feed each individual polyps as they are in fact seperate organisms and not part of the 'Borg Collective', so to speak.