This patch of hydroids came as a hitchhiker on my Live Rock. At first I was just sooo Happy with myself for finding such a wonderful piece of rock. Later, I found out that hydroids are nearly as bad as Aiptasia, which you can also see in the pic. I have been working to rid my tank of these pests and with the help of peppermint shrimp, I am glad to say the Aips. are under control. The hydroids prove to be more difficult.I pull them off when I get the chance and have also used the Kalk paste method of control.