how old is tank? what else are in there?
my experience with these branching hydroid is that they can "come and go" in monthly cycles but once they are in your tank, it's going to be impossible to get rid of them b/c the typical CUCs do not touch them. They spread quickly and can sting corals if they spread too much to a coral. They feed on dissolved organics instead of nitrate/phosphate.
My tank is 4 years old, very mature and stable. These would pop up whenever i feed my corals too much (just went thru another round last month when my Vibrant treatment caused too much dissolved organics). My other filter feeders (feather dusters, sponges, vermetids) would catch up and out compete the nutrient so they'd then go away on their own after a few weeks. This is probably one of the only ways to keep them in check once they take a hold in your tank - they reach a natural equilibrium with the rest of your tank.
In young tanks it will be harder to control this way b/c you won't have the necessary bio diversity in your filter feeder population. In this case really the only way is to either starve them out, use aggressive filtration to remove desolved organics, or a "hit or miss" scourch earth medicine that will also kill off your biodiversity (forgot the name, google "hydroid treatment" and it's the only name that pops up)
Depending on where you are in the above, might be better to just toss the whole thing out if you can't carefully and tweeze them out of the frag/hammer. a quick search on these forums and you'll see a lot of pain stories on how to deal with hydroids - the worse ones will be of the "colonial hydroid" variety. Your branching hydroids arent as bad, but I think most would find them still a headache to deal with due to its fast spread rate and a lack of natural counters in our tank.
