The white poop is probably a reflection of the fact that she is not eating. The dark part of the poop comes from what they eat and this appears to be missing. The white part is the equivalent of urine.
Overall, I'd guess that she is not eating (and therefore not pooping properly) because she has an abdominal infection that originally came in via the umbilical cord. That hurts, and she is not hungry.
If you find that she continues to get worse, you will have to think about what to do. If it is an abdominal infection then it will become very painful. Of course, we are not 100% sure that an infection is what it is, but it is looking pretty likely.
She would have caught any infection in the hours after hatching. The fact that the yolk etc took a long time to be absorbed makes infection more likely, but for future hatches also keep in mind the need to make sure the incubator is completely clean and disinfected and that you only set completely clean eggs. Wash your hands before handling newly hatched ducklings. All these things help to keep infection to a minimum.
