Lichen striatushttps://en.wikipedia.org/wiki/Lichen_striatus
Lichen striatus is a rare skin condition that is seen primarily in children, most frequently appearing ages 5–15.  It consists of small, scaly papules. The band of lichen striatus varies from a few millimeters to 1~2 cm wide. The lesion may range from a few centimeters to the complete length of the extremity.

○ Treatment ― OTC Drugs
Some patients of lichen striatus recover within a year without treatment. If it continues for more than a few months, please consult a doctor.
#Hydrocortisone cream
  • 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.