What are you feeding your ducklings? If they are getting chick feed, are you adding niacin or brewer's yeast to their food? Unmedicated chicken feed is ok for ducks, but you must add either niacin or brewer's yeast to their food for them to develop normally since ducks need that. Niacin deficiency usually shows up initially as leg problems, but they can usually be fixed by adding niacin or brewers's yeast asap.
If that isn't the issue, I would just try to let the little guy rest the leg as much as possible since there is no sign of injury or swelling (is that leg warmer than the other?). If there are others who are possibly knocking this guy around or walking over it due to the leg problems, you can try separating this one where it can still see the others. Could just be a sprain or strain that will heal on it's own after a few days or so with plenty of rest.