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You need to tell me the exact look of the patch .Is it milky white in color or it is just a shade lighter than your normal skin color.If it is milky white then it is vitiligo
) .And if it is just a shade lighter then it is called PITYRIAISIS ALBA. P.Alba is a benign condition which is because of either excessive use of soap or dryness or excessive exposure to sunlight.The skin is usually dry in such a case.Here you need to apply a good sunscreen daily three times and use a moisturizer over face.Also use a moisturizing soap for face.
If it is vitiligo then you need to apply a mild steroid
cream over the spot and use some photo sensitive cream with sun exposure .Its a long term treatment and you should consult dermatologist
for proper diagnosis and management.