Oh dear! I don't know much about Ducklings except that they are CUTE!
To me it looks like an unabsorbed egg sac. It's fast approaching being like that for 24hrs?
I would try to keep that tissue moist - coconut oil, bacitracin, plain neosporin. Place it in a small container lined with something snuggly and put her back into the incubator to keep her warm.
I would worry if it is able to be absorbed, it can turn into Omphalitis
https://www.backyardchickens.com/articles/mushy-chick-disease-yolk-sack-infection-omphalitis.64686/
The longer it stays like that, the less chance she will survive, but I don't know of anything else you can do.
Hopefully someone like
@Pyxis will chime in.