Some birds have more dander than others. Of course cockatiels are a powder down bird, so what you are seeing is the powder they distribute on their feathers to protect them, instead of the oil that most birds have. You should not bathe or mist a powder down species too often because this can actually cause damage or cause them to over produce the down. You can’t get rid of it because it is essential to their health. For some reason she seems to have an excessive amount of powder. This could be natural to her or could be due to an imbalance. There are diseases & disorders that can cause this, including viruses that some birds can carry. I would recommend taking her to an Avian Vet for a complete checkup and possibly virus testing.
Thank you for asking Lafeber,