Another cockatiel was plucking this bird, so i separated them, now im wondering if the bird has mites, or feather disease, its been about 3 weeks now, its back and chest of the bird look like this, as if it was cotton
The cottony feathers you are seeing is the down, the birds outer feathers have been removed. Are you sure it was the other bird doing the plucking? Or is this bird self plucking? As Enigma asked, did the bird look like this when you separated? Or did it get worse after? If it got worse then I would say the bird is plucking itself for whatever reason.