Nutritional deficiency: A lack of certain vitamins and minerals, such as vitamin A or calcium, can cause a budgie's beak to become soft and crusty. Make sure your budgie is getting a balanced diet with plenty of fresh fruits, vegetables, and a high-quality pellet mix.
Yeast infection: A yeast infection, such as candidiasis, can also cause a white crust to form under a budgie's beak. Yeast infections are often associated with stress, illness, or immune system problems.
Trauma: If your budgie has injured its beak, it may develop a white crust as part of the healing process. Monitor your budgie's beak and make sure it is not excessively damaged.
If you have any concerns about the white crust under your budgie's beak, it is best to consult with an avian veterinarian or budgie expert for a proper diagnosis and treatment.