The cream color can very well be just a feed thing. Solid white ducks (c/c) can vary from very white to a creamy off-white to even almost a buttery yellow. The color popping up just means that some Saxonies are heteroygous for white (C/c). That said, if your goal is to breed Saxonies that only look like Saxonies, I would not breed from the White. If she gets paired with another split white (what her parents would have been if they looked Saxony), 50% of the offspring would be white. You probably don't want that (unless of course, you want to start a line of white Saxonies).
As far as molting, that shouldn't matter. She should remain white. The thing that would concern me the most is if she really does have a distinct neck ring and it is not just an optical illusion or something (so hard to tell from pictures). The recessive White is a problem, but if it is not recessive white it could be an even bigger problem. Once your birds are feathered out in adult plumage, we can tell more. Seeing pictures of the other person's adult breeder ducks would help too. If she is just extremely lightened because they have brown or buff dilution in their Saxonies, that is very hard to decipher and eliminate.
Editing to add- Now that I look at all the other birds in the picture, it does worry me a bit. It will be interesting to see how they feather out, but even incorrect genotype Saxonies are gorgeous so I am not sure if matters to you if you are breeding for fun or for pets. Keep in mind, the genotype we use for Saxony has also changed over time. Holderreads have gone by at least two different genotypes over the years, so it is no surprise that they vary (currently wild type plus Bl/Bl li/li is the accepted correct color).