Several things.. If this is something new or sort of reccuring in warmer/summer months and fade over winter, occuring on chest/back etc..Likely tinea versicolor (fungus). Can be eassily treated, but reucurrence is common. Vitiligo (autoimmune), pityriasis alba (probable viral) are relatively common as well..Sometimes steroid injections/potent topicals can cause such as well.. Consult doc.. Good luck.