Hi there and
It looks like the ducks umbilical cord.
If you put the duckling under the brooder or heat lamp
The heat should dry it up and make it fall off.
Don't cut it as you may make the duckling bleed to death.
I breed and show Gold partridge pekin bantam Coronation Sussex bantam Barbu d'Anver quail colour Black bantam silkie Cream legbar Rouen ducks