Hello,
It can be due to pityriasis alba or pityriasis versicolor. Pityriasis versicolor is caused by an overgrowth of a yeast fungus and is found mainly on sun
that usually resolves by adulthood.Severe cases may benefit by Psoralen plus ultraviolet light A photochemotherapy (PUVA).
If the symptoms are severe then please get it evaluated from a dermatologist.
I hope it helps. Take care and regards.