What Is The White Dart Under The Foreskin Of My Penis?

Hi! Not sure which topic this comes under as i was told it s not a sexual transmitted disease however i d just like confirmation from you as to what it is exactly? As mentioned i visited a clinic around 4 years ago and was told it wasn t a sexual disease just dirt under the foreskin! It does just wash off nice and easy but lets say i don t clean it everyday.... after 3 days it does start to look like more then just dirt! White dirt! I know the dirt probably comes from urine and sperm but some confirmation would be appreciated so that i can stop worrying about it. Thanks