I had a couple of rescue ducks who were fed completely on cracked corn.
Their white feathers were very yellow One had a neurological issue that righted it self somewhat when I got them and started feeding them properly.
One had a neurological issue that righted it self somewhat after a few weeks of proper diet. She would stand and circle in place.
Now you have to herd her a certain way and she only circles when she is upset about something. We call her our "democrat duck" because she doesn't walk straight, but veers off to the left.
I know i am late to this but I would agree that the corn was likely contaminated somehow.
A duck would be fine eating only cracked corn for a few days.