Oh dear!
That is the yolk
This baby is very premature
Ducklings take a very long time to hatch, it looks like this one needed a couple days.
I'm sure I'm saying what you already realise, but please please make sure your child can't do this again, not sure how old they are, I understand it can be tricky and I'm not blaming you
Place the baby (leaving him in the remaining shell), into a cup lined with paper towels, very gently. And leave it in the incubator in hopes of him absorbing the yolk. This is possible, but usually the yolk is far smaller in that situation.
I would prepare yourself for this one not making it. He may surprise us, but that's a lot of yolk left, so just be prepared for the worst.