Could be she rubbing it on something and the hair is coming off. Another thing is an allergic reaction to something that is on her face.
Is she scratching a lot? Have you introduced anything new to her cage?
Some mites are microscopic and you can only see them with a skin scraping. This seems to have come on very quickly. If there's nothing new in her cage (food, housing or bedding or cleaning solution), keep an eye on her, see if it continues to spread. She'll have to see the vet then.
The Content on this Site is presented in a summary fashion, and is intended to be used for educational and entertainment purposes only. It is not intended to be and should not be interpreted as medical advice or a diagnosis of any health or fitness problem, condition or disease; or a recommendation for a specific test, doctor, care provider, procedure, treatment plan, product, or course of action. Med Help International, Inc. is not a medical or healthcare provider and your use of this Site does not create a doctor / patient relationship. We disclaim all responsibility for the professional qualifications and licensing of, and services provided by, any physician or other health providers posting on or otherwise referred to on this Site and/or any Third Party Site. Never disregard the medical advice of your physician or health professional, or delay in seeking such advice, because of something you read on this Site. We offer this Site AS IS and without any warranties. By using this Site you agree to the following Terms and Conditions. If you think you may have a medical emergency, call your physician or 911 immediately.