Hello,
It can be due to seborrheic dermatitis. Seborrheic dermatitis is a common skin condition that causes flaky, dry, white to yellowish scales to form on oily areas esp under the nose or anywhere on face, neck, scalp or upper trunk.
Topical antifungals and mild steroids are the usual treatment and combination of the two can be used to treat stubborn patches. Oral antifungal drugs and immunomodulators such as tacrolimus and pimecrolimus are used in very severe cases. The other possibilities are of eczema or dermatitis. Best would be to consult a dermatologist to confirm the diagnosis.