It could be a fungal infection
of skin known as pityriasis versicolor. It is usually asymptomatic and is usually localised over chest and back. At times face, neck or arms could be involved too.
To treat the same you may apply ketoconazole
shampoo over the affected area for 10 minutes and then wash off. You may also apply any ntifungal cream like clotrimazole
, terbinafine etc to treat it.
As dermatology is a visual field
, it is always better to get a definite diagnosis prior to starting the treatment. Hope that answers your query. Take care.