They spawn, and then the babies land on other rocks and form a small polyp, and then bud from there. I think it has something to do with the moons personally. But once they cover the whole rock or so, then theyll spawn. I had a large sun coral in my tank and I sold it, but about 2 weeks later I had baby sun coral polyps in the lower flow areas of my tank.
Its been about 4 months since and they havent grown, but I havent been spotfeeding them.