Because we aren't vets we can't make a clear diagnosis but since your duckling hasn't been on any type of niacin supplement I'd start there. Usually a week or longer gives you an idea that is what it is. We have to eliminate and when they are young like yours it's best to start with the niacin deficiency. Hopefully, that is what it is because it can be easily fixed when they are young. I'd get this duckling on it ASAP.
Activated charcoal is always good to have on hand.
If this was a toxin I don't think your duckling would be eating. Does he/she have a good appetite?