The way White works is, one copy of white makes a duck pied. Two copies makes it solid white. So, in order to produce solid white, each parent has to be at least pied (carrying one copy of white). Of course the parents could be white as well, but they would definitely have to be at least pied. I'd bet another duck laid an egg in their nest, and they weren't actually the biological mothers of the white duckling.
Do you have other female muscovies? Depending on what colors you have, it's possible that it would be easier for you to breed blue fawn babies using another of your ducks instead of the white female.