If mine I'd give it time to get its legs working. A couple of years ago my Muscovy had 2 eggs she was brooding on and my hen had 2 duck eggs she was brooding on. 1 egg from each one was not viable so each hatched one duckling. I decided to give the one ducklings that hatched under my hen[chicken] to momma Muscovy she took it right in.
I'd try with your mama by end of day if the duckling is looking stronger you just don't want it to get stepped on by mama. When did it hatch? this morning? I'm thinking you are not in the USA? time difference.