the cave is not necessary for it. sun corals (tubastrea sp.) are a non photosynthetic coral, which means that they have absolutely no light preference because they do not feed from it. this also means that they do NEED to be fed, but on a good note ... they can be "trained" when to come out if fed the same time everyday, or for them to always be out ... make sure there is always something in the water column for them to eat.
they are a fairly hardy coral species,
IMHO. i always fed mine mysis, brine, and cyclopeeze or the blender much that i prepared for everyone in the tank. it is much easier to spot feed them if you cut the top off of a 2 liter bottle and set it over the coral with the small opening facing up and squirting the food into it. this keeps the fish and inverts from stealing the corals food and gives the polyps time to eat.