Mostly breeders had one male with two to three females so it's not fair to the younger one so you can either take him to the center place or separate him or even put one of the females with him till he is full grown unless he's not
Well the tealish green coating is just the black its normal on black ducks which they are adult feathers so there maturing! It is possible but it's up to how fast there maturing, males will hiss and females will make a peepish tone also males will have a curled feather on the top of the tail...
You probably already made a pen but a tip if you are going to make more is to build I frame with 2\4 and use a staple gun to staple fence on the inside then nail or screw the frames together. Hope I helped!