The scaleless fish like the Powders can get many skin irritations that look like ich but aren't....but since it has spread you have a parasite.
For so many years I never worried about QT except for things that have a tendency to have worms...any signs of parasitic disease and you just crank down the salinity for a few weeks...problem solved.
When my Powder Blue got some spots and they spread to my Anthias and I found out that Hypo was more deadly to corals than to Protozoa, I had a problem. My PB continued to eat like a horse but was gradually getting thinner and thinner. At that point my usual "if their eating let them fight it" was proved wrong...So fish trap and Hypo in the QT tank it was. All the fish I could get out went into a spare tank and took a couple of days to move the salinity down. (the trip back up in salinity has to be much much slower) within days all the fish had improved (even the ones that didn't look infected) and were spot free and gaining weight back.
I made the choice not to rip the tank apart to get the last goby who had no visible signs as ultimately I was not willing to risk my corals. So technically he may well have carried an invisible culture of ich in his gills that could come back in future, but some sources list Gobies as being effectively immune and unlikely carriers.