Are you removing the rock to treat it?
Fenbendazole is a dewormer. Along with the hydroids, you will also be killing any bristle worms, spaghetti worms, and filter-feeding fan worms in your tank. You could very well do more harm than good adding that to your display tank...the die-off could potentially cause a large nutrient spike in your tank.
If you want to remove the rock, place it in a hospital tank, and treat it...then it may be an option. But considering they are irritating your zoas it's probably safe to assume that they share a rock with your livestock. I'd recommend against going that route and just try to spot treat instead.