wow, thread back from the dead! I broke down and cooked my rocks that were infested with hydroids for 4 weeks in hypo-salinity (1.009). It was drastic, but it worked. It killed all other inverts on the rocks (worms, dusters, etc...) but the coralline was still thriving when I replaced it back into the the tank.
It is a drastic measure, but just know it is a solution if you can get the corals off and remove the rock for a few weeks. a couple months later and the rock is rebounding with life and no signs of those b#@%ards!