Clear as day it is ich, he is the only one with it as I can see it came out of nowhere.Isnt ich in the system now. I think you'll have to take all the fish out for QT. Just keep it happy and feed. Mine loves algae sheets. Why do u think it's ich. Is it clearly tiny specs of white dots? I've never QT before and have no idea how. Ha.
Is trigger still eating? If yes keep that going, and he should just come around.