FYI: Bristletail Filefish eats colonial hydroids
 

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". :)

If only you could spray paint them with some color

Thats great to know. I had a good friend get out of the hobby after his tank became overrun with hydroids... At one point in his battle he blow torched all of his rock trying to kill them off.

not to hijack, but at one time seemed my rock was covered in hydroids. Tried pulling them out but didn't help so just left alone, now need to look really close to find any. Thinking one of the phases like diatoms.