What Is The White Smooth Substance On Penis Tip Which Is Unable To Wash Off?

hi, im currently 18 and i've started masturbating from the age if 16, thou not obsessively, about 3-4 times a week, i've started seeing white smooth substance on the tip of my penis, at the start, it was easy to remove ( wipe off ) but as of recently, it has escalated to an extend that i could even see white 'stains' when i pushed down the excess skin, and i couldnt wash it off in the shower like i used to.. im afraid it would lead to other diseases so in writing in to ask for diagnosis/solutions, thanks in advance, mike