Vermetid snails? I'm new and have no idea about corals, but they are pests that kind of look like little tubes and they shoot spider web stuff out to catch food. Could it be some of these guys bothering the coral? Like I said, I'm new so someone that knows will either tell you to pay no attention to me LOL or help you diagnos for real. I just thought it would be worth mentioning when you said spider web stuff coming off of it.