Lichen striatushttps://en.wikipedia.org/wiki/Lichen_striatus
Lichen striatus is a rare skin condition that is seen primarily in children, most commonly affecting ages 5–15. It consists of small, scaly papules. The affected band is typically 1–2 cm wide, but may be as narrow as a few millimeters. Lesions can extend from a few centimeters to the entire length of the extremity.

○ Treatment ― OTC Drugs
Some patients with lichen striatus recover within a year without treatment. If the condition persists for more than a few months, consult a doctor.
#Hydrocortisone cream
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  • The white linear patch above the black patch is the lesion of Lichen striatus. The lesion mostly appears as linear erythematous grouped papules or patches. The black patch is a café-au-lait macule.
    References Lichen Striatus 29939607 
    NIH
    Lichen striatus (LS) is uncommon and occurs most frequently in children. It presents as a pink rash with raised spotting that comes together to form singular or multiple, dull-red, potentially-scaly linear bands that affect the Blaschko lines.