I too initially thought that its the dead skin getting wet or digested by saliva, but when I carefully observed a thin white layer covered the entire innerlips, inside of cheeks in the morning before brushing. After brushing mucus membrane looks normal. I strongly think that the white stuff is forming inside the mouth as on lips, but is removed because we tend to eat some thing through out the day. My doubt is a thin candidal biofilm on entire oral mucosa that extends a bit onto the lips which is "our condition". Try to observe a thin coating on your oral mucosa before brushing.