Hi, I had gone through your question and understand your concerns. Well, this white substance is probably semen which is normally ejaculated during masturbation. In rare cases as a part of STDs it may be pus but then it would bi discharged spontaneously. Certain degree of pain is expected and depends on intensivity of mastrurbation, but if there is significant pain then it may present uroinfection or prostatitis and in this case you should do urine analysis and prostate ultrasound. In ever ycase, I dont think this is a serious problem. Hope this answers your question. If you have additional questions or follow up questions then please do not hesitate in writing to us. I will be happy to answer your questions. Wishing you good health.