If anyone is battling a case of colonial hydroids, a nice surprise I came across when I introduced F'ugly the Bristletail Filefish (Acreichthys tomentosus) aka Aiptasia Eating Filefish is that he eats them. Cleared my tank of them in a month or so. I decided to introduce him as I find Filefish to be an interesting addition. My tank doesn't have any Majanos or Aiptasia (which they are known to eat), so I don't know if he has appetite for them as well. He doesn't touch any of the corals, clams, or anemones in my tank either. Maybe I got lucky with F'ugly's possibly unusual appetite, but it's worth a try if you're looking for something to eat the hydroids. Bonus is these Filefish unlike some other species are considered "reef-safe" and only get to about 4.5".
