I got my ducks from Metzer 3 weeks ago and one of the Pekins had a few of these episodes. I offered it water and wet its bill and after a while the situation resolved itself. Mine arrived happy and healthy too.
That nodule is the ducks oil gland (uropygial gland). They nibble and rub it with their bill and then spread the oil on their feathers to waterproof them. The drake feathers won't come in until they are getting their adult feathers.
Hello fellow duck people. This is normal. When I was a kid and had my pekins the females used to do this. I found a video on Youtube showing this behavior. It makes me laugh every time I see it.