This is called leukopkalia, which is a pre-malignant lesion. It most likely will turn into cancer if you continue to smoke as it is from the tobacco and other chemicals in the cigarette. Not the nicotine though.....
My dentist discovered this a few months ago and sent me for a biopsy. Luckily, I was okay and half of it went away in just 2 weeks when I stopped smoking. I was using the e cig with nicotine and it did not aggravate it or anything. I also asked my doctor and he said that it is not the nic that he is worried about but the tobacco and all the other crap in the cigarette, he also said the e cig is a wonderful way to quit smoking.
It has now been 2 months that I have stopped smoking and about 75 % of it has disappeared. Though, my doctor told me that my chance of getting cancer are higher because of this.
Oh by the way Im under 35 years old. I also thought it was thrush, my doctor as well--I was misdiagnosed, luckily my dentist took action.
Hope this helps

Please take this seriously. If you have any questions please feel free to contact me!