I agree with you, it looks like new feathers growing in. The sheath will break down and leave flakes on the feathers. If they are on the wings they should be able to groom them off when they are done growing. I do assist when it's on the crests because they can't groom their own heads, and it can draw in pecking from other birds.Some people groom them by helping to pick the shafts off. I believe it was @oldhenlikesdogs who educated me about this. Have fun with your birdies!