This is also known as tinea versicolor and is a common fungal infection (caused by Malassezia). Small, scaly patches of skin became hypopigmented (or sometimes hyperpgimented). Often the smaller patches join together over time.
It most commonly occurs on the trunk, arms, and neck. It is usually asymptomatic but is occasionally itchy.
Diagnosis is clinical but is can be confirmed using a Wood lamp, a skin biopsy, or a fungal culture
The treatment is with topic antifungal cream or shampoo (e.g. ketoconazole) if practical – usually for up to two weeks. Or if this fails, oral antifungals (ketoconazole or fluconazole) can be considered. The colour can take a few months to return to normal.
References
https://www.nhs.uk/conditions/pityriasis-versicolor/
https://wikem.org/wiki/Tinea_versicolor
https://dermnetnz.org/topics/pityriasis-versicolor/
https://www.bad.org.uk/shared/get-file.ashx?id=226&itemtype=document