Hi,
If the chick is weaned, then you need to separate it from the parents. You can’t let independent offspring remain with the parents or this is the type thing that happens. The plucking will escalate to aggression, and if it’s a female, the male will mate with his own chick. If it’s a male, he may kill it because it is a rival. Separate the chick, and see if this makes a difference with how the male treats the female. I would move the chick to another room. Clearly the male is upset that the chick is still around and he’s ready for it to be gone. In the wild, the parents chase the chicks off as soon as they are weaned. Now if this chick is still being fed by the parents, make sure you have removed the nest box if the chick has left it. You should always remove the nest box as soon as any chicks have been weaned, and you should always remove chicks from the cage as soon as they are weaned.
Thank you for asking Lafeber,
Brenda