There's a local scientist who claims he has done studies on this. I have tried it and it seemed to work. I had several fish in my 125 with ich bad enough it killed a couple. After several outbreaks in a couple of months I tried this guy's idea. His study claimed that aptasia eats ich in its swimming/floating phase. I therefore added quite a few aptasia to my sump and quite a few peppermints to my display tank. Soon I had an aptasia farm in my sump and no ich in my main display. I was ich free for about 6 months until one day I broke a thermometer (I was worried it might be mercury) in my sump. I pulled the sump out and replaced it the sump. Unfortunately, I had no more aptasia after the replacement. In the weeks that followed, the ich came back. I added more aptasia and the ich was gone again.
While I am not sure it would work in time for your powder blue, I would trap him and put him in copper ASAP if it were me, I would still try the aptasia thing to get your main reef with less ich. There is really little to lose as both aptasia and peppermints are "reef safe." FYI, I tried cleaners and everything else everyone here recommended before the aptasia thing, but had no luck.
If anyone else has had experience with this I would love to know it.
thanks
p