I'm guessing yellow colonial polyps.
GASP!!!! What happened to your lovely avatar?
Heres something I just thought of.
Get a box of baking soda and bake it.That will give it a high PH.Then mix a couple of teaspoons full in a glass of RO/DI water.Then use a hyperdermic needle and shoot a few of the hydroids up.The combination of the baking soda buffer along with the high PH inside the animal may kill it.
Just a thought.