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The white powdery substance coming from inside skin of penis head(Glans penis
) is called smegma
which is a deposit formed due to retention of few drops of urine and even semen in people who have not had circumcision
;it is normal to have it but you should make it a practice to retract the skin and clean the powdery substance with water and if possible with soap so that there is no infection.As for the black substance coming from scrotal skin it may be collection of salt;due to sweating mixed with infection if there is itching.If it is troubling you can consult a doctor in person so that he can examine you and advise you the required treatment.