Be careful with that stuff if you get it on you it won’t come out and if you get on your hands they will be purple for quite a while. Of course so will your Pekin be purple but it is good stuff.
Have you looked for mites? they will be black and smaller than a pepper flake . They are very fast too. Ducks don't usually get external parasites if they have a way to wash in a pool etc.
Feathers coming in can be itchy but that looks extreme. like something other than feathers has irritated...