Unless the duckling is suffering or in obvious pain, I wouldn’t cull him yet. It seems like others have had hatches like this one and they’ve turned out just fine.
I'd place it in a margarine container and leave it for hours till it finds it feet. Didn't look like an infection but that the yolk was not fully absorbed. My Duckling yesterday hatched that way and is up running around eating ..
Thank you all so much for your help. I let the baby go, out of sheer ignorance it seems. My heart is broken, I tried so hard to help this little one. I've lost a few ducks, but this has been the hardest, and now to find out I made the wrong choice, I'm devastated.