My drake

Good Morning,
Please I need some help. I am a new duck/chicken owner, and I'm crazy about them. I brought 3 Rouens on April 30th which makes them about 4 1/2 months old. Then a couple of months ago I rescued a drake (not sure what kind) that is about 2 years old. The last few days the drake has been going after one of the
Rouens and keeping him/her away from the flock. I don't want to upset the natural balance, but I'm worried about him hurting the young Rouen. I am able to separate them if needed, but just not sure. If anyone can give me some advice I would sure appreciate it.