Sorry, I'm a little late on this one but I'd like to throw in my 2cents.
I don't believe you can over feed them but you can feed so much that you 'pollute' your tank (as Melosu says).
It sounds like the polyps have receeded a bit and it will take some time for them to fill back out. When I feed my sun polyps I simply soak the mysis shrimp in garlic and vitamins. Rinse them off again and feed each polyp a few mysis shrimp each. (Also, I feed the tank about 10 minutes before so they are expanded when I go to feed them).
Since my other fish are pigs, I cover the sun coral with a plastic bell (fashioned from the top of a 2 liter pop bottle) so they can't rob the polyps of the mysis. I leave the plastic bell on for about 30 minutes....which gives them more than enough time to eat the mysis.