I rescued a dying clump of Sun Corals as well... I've only been feeding it pellets though as anything else is difficult. Either it floats away or it gets stolen by the fishes and shrimps. I'm too lazy to use the Tupperware or cut plastic bottle method.
What I've found is that once the water is "dirty" some of the polyps will extend. I then drop a few sinking high quality pellets into the polyps and within a few minutes, ALL the polyps will bloom... day or night. I also keep them in a bit of shade. Here's the clump just starting to expand.
