I haven’t seen this in any anime I actually watch, but I mostly watch shonen and other popular anime, not the romantic or sexual genres where this seems to come from. But on the internet, I see memes and art of anime girls with a triangular flap of skin hanging from the upper lip, usually on the left side (from her perspective). It’s usually on characters who have more of the “manic pixie dream girl” personalities, which is I guess why I haven’t seen it on the TV.
Here’s an example I saw on MULTIVERSE today: 
Normally there’s a bit more effort to make it look like a tooth, but I guess at this point a vague squiggle is shorthand that can be recognised to those familiar with it.