Idiopathic guttate hypomelanosis https://en.wikipedia.org/wiki/Idiopathic_guttate_hypomelanosis
Idiopathic guttate hypomelanosis is a very common acquired disorder that affects women more frequently than men. The condition presents with skin lesions that occur primarily on sun‑exposed areas, suggesting that sun exposure may play a role. Similar lesions can appear after long‑term treatment of melasma with the QS1064 laser. No specific treatment is required.

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References Idiopathic Guttate Hypomelanosis 29489254 
NIH
Idiopathic guttate hypomelanosis (IGH) is a benign, typically asymptomatic, leukodermic dermatosis of unclear etiology that is classically seen in elderly, fair-skinned individuals, and often goes unrecognized or undiagnosed. Occasionally, IGH is aesthetically displeasing. However, it is not a dangerous process. Once present, lesions do not remit.