I think you can and should certainly address nutrition first, but I will share that I had one out of my Khaki Campbell hatch that did not feather in normally. It was not pink skin exposed tho, so I am not sure what's going on in your case. After it was really obvious that this duckling wasn't developing at the same rate as the others, I conferred with other experts here and ultimately decided that culling was the best option. What I'm trying to say is that here where it gets cold, a duckling that isn't well feathered is not likely to survive. Speceider did tell me that he has observed the phenomenon in the wild population as well, so that could be a sign that genetics are affecting feathering processes and rates... Meaning that you likely do not want that duck reproducing in your flock. Ultimately you will need to decide what is best for you and your flock. Do start with nutrition and observe, hopefully that will sort itself out quickly.