It's fungus growth..it will go with medicine.. for detailed prescription do direct online consultation

Pityriasis versicolor. It is a common yeast infection of the skin, in which flaky discoloured patches appear on the chest and back. Pityriasis versicolor is more common in hot, humid climates than in cool, dry climates. It often affects people that perspire heavily. It may clear in the winter months and recur each summer. Medication is available to control and clear this. Consult a Dermatologist for the same.

Looks like a infection