Yeah, it might be a Muscovy mix, especially the way the coloring is coming out and the head. But, I had a weird mallard-dy duck with a skinny body and long legs, too. Maybe yours is a Muscovy-mallard cross.
I say appleyard or rouen cross. The body type is completely wrong for a muscovy, even a mixed one.
The coloring indicates the duckling will likely feather out with mallard-like patterning. The black stripe on the head makes me think appleyard, though definitely not a pure one