Hi. Greetings. This could be tinea versicolor.. This is a fungal infection caused by pityrosporum fungus and is not anything serious. Usually subsides with short course application of anti fungal creams / lotions. However marks may take few weeks to go. Kindly get this confirmed with an examination by your dermatologist and obtain prescription from him. Or you could send me pictures. Hope this helps. Warm regards. Dr Govind S Mittal.