What duck has green head and black body?
A male mallard in breeding plumage.
There is no wild duck with a green head and a black body.
Mallard drakes have the green head, but the body is bascically buff
and brown, with white and a blue or lavender speculum patch on the
wing, with a dark brown or chestnut colored chest. The bird you saw
may be one of many hybrid species, as for example I have seen seen
domestic ducks that have bred with mallards to produce many
different colors, including basic black birds.
In exotic species, male Indian runners and crested cayuga ducks
are black with green heads, and are allied to mallards. But no
North American species.