Have you tried to feed it yet? To get the polyps to extend, i would put some selcon or zoecon in the tank, that should getb it to extend. Then squirt a little food on it.
I would take it out from under the cave. They dont need light but light wont hurt them, thats a myth. It makes it a million times easier to feed if its out in the open.
There is a sun coral thread a few threads down on this page where myself and a few others describe their feeding techniques. Check it out.