The one i get when i start licking my lips (I do my best to stop it but sometimes i gotta do it, liplicking is human lol).
Or the one i get when some skin is growing, from EC, near the area where my lips meet and then it gets easily white when i drink or talk etc....
But for me, 90% of time i have white gunk because of liplicking and it is simply accumulated dried saliva so there is nothing to worry about....it goes away whenever i eat or take shower.