I have never seen anything like that on mine, but if I ever feel the SLIGHTEST "greasiness" like ejuice, I wipe it immediately, inside and out; I don't want ejuice in my mouth unless it's in vapor form! So maybe that keeps them clear of anything like that "crust." Ewww. And mostly I use acrylic tips, sometimes glass which requires even more frequent wiping because I can *see* all that condensate building up inside. The one metal I use pretty much daily, a "gun metal" chromed stainless, I do notice that sometimes it doesn't feel quite as smooth as it should, so I wipe it carefully with a wet papertowel, then with a dry one. It fixes it.
I noticed many years ago, however, that my "smoking" technique was very different from most peoples'. I have very thin lips, so maybe that's why, but my cigarette, and now my driptips, do not ever go inside my lips; I hold them to the outside of my lips, always. My filtertips were NEVER wet, and if I happened to slobber on one, I put it out immediately because that's just gross. I feel much the same about my driptips, but thankfully, they can be wiped and dried off.
Andria