They do that when they are stressed... So the light change probably didn't cause it, unless it happened shortly after you changed the light (not a week after).
Did you add anything, or stir something up? Maybe chemical warfare between corals?
Another possibility.... Now that I think about it, is that its looking for food... I've heard of some corals expelling guts so they and using their guts to grab at near by food that they couldn't reach by normal means. But I don't think this happens very often at all.