Question

To whom does Macbeth refer when he says what seem'd Corporal melted as breath into the wind

Answer 1

You have to look at the lines preceding the line in order to know who he is speaking of.

Whoever it is apparently dissolved into thin air. Then you can see there is a stage direction that states "Witches vanish." This is who Macbeth is referring to when he makes that statement.

Answer 2

hmmmmmmm.....Malcolm maybe?