Hi,
First off ,there really isn't a need for a collar, as he/she isn't mutilating themselves.
Your new bird has gone through a lot , changed homes and routines, diet , and added to a new bird . And then 2 weeks in a different home for pet sitting..
As mizzly said,these are the grey under fluffy feathers and he or she has plucked out the color feathers.
Feather destruction behavior is complex. Agree 100% with everyone that the place to start is a health exam with your Avian veterinarian. Plucking can be a symptom of ns y different health issues , so you want to rule that out.
After ruling out health, then it's behavioral. This pattern of destruction is pretty typical of behavioral,.
If you have these 2 caged together, I would separate to side by side cages ,and still time out of together tho. Observe and monitor. Very often one bird is stressed by sharing a cage . It can be very subtle to pick this up . I think it's a very likely scenario and a place to start. They can still be very good friends and enjoy time out together, but the new one might feel more confident and comfortable with their own space for now. In the future they may be happy to share a cage again. I previously had 2 very bonded females who shared a cage , but I over r their years together, occasionally needed to be separated to side by side, and then would choose to share again. It was very subtle, Ta-dah would displace Burt from preferred spots, and from food dish . They did not fight . Still preen each other.
When you move them to side by side cages, mashed up together . Observe, from a distance, try nit to be obvious and intense about it. Give the observations a few days . While still having them out together as your normal routine. If either bird seems indifferent to this new situation, and is happy to chill out do their own thing , tho being interested in what the other is doing is fine, then this is probably for the best for them. One of them may be frantic and contact calling , and worried about not being attached at the hip, but if the other is not, then they should have their own cages.
I suggest also adding back 10% seeds to the diet, you can offer at the very end of the day and thst way should not affect pellet or veg consumption. These are hook bills, and if Zazu had seeds before could be feeling the frustration.
On that note as well , I would be weight checking. I hope you have the weight when first arrived to compare to. But if not , you can see where it is now and how that compares to the average GCC expected weight. I like once a week or once every 2weeks for life .. daily if I think there is a health issue or changing things like diet and changing cages. All to often quick diet changes can have the bird not actually eating enough, even if observed eating. Or stressed by the change , and even health issues even though pellets s d veggies are healthy food , sudden changes can disturb gut flora, trigger fatty lipidosis as less calories less fat , triggers sudden release of fats ( lipids) that can overwhelm or inflame the liver . I know that sounds weird , but it's how the body works.
It's normal to never actually see the feather destruction happening, they are sneaky, and often it happens at night or wee hours of the morning. As shared if you do see it, pay it no attention whatsoever. Birds do a lot of preening normally a d you don't want to disrupt that. They especially seem to like doing a big preen session even out of the cage or hanging out with you.
Be sure to offer a bath daily, as free choice. GCC seem to bathe nearly daily. At least mine all have. I use a sturdy ceramic bowl with a thick rim , and only about 2 inches of water in it. One is a rabbit food or water dish , the other I have is a ceramic pie or cake dish, as well as one of plastic clear bath dishes they sell for parakeets or small birds.
