While playing the violin, I tend to make all sorts of strange faces. Like, faces that involve muscles I didn't even know I had (although now I have this strange ability for bizarre faces, which comes in handy when you're doing childcare ;-)).
For me, it manifests itself in the form of strange eyebrow movements and rapidly blinking eyes. I'll see videos of myself playing while my eyes are batting, and no, it's not particularly attractive.
I've started to curb this habit though. I received some advice in a different question, and have started thinking about it more. It is suggested that people who's facial muscles twitch a bit may be nervous or, in my opinion, straining to much. You should be totally - totally - relaxed while playing. If you are stressed, it will show in your body language.
Try recording yourself again and again, and focus on staying calm and collected... cool as a cucumber. If you get comments from non-musicians about your facial expressions, invite them to try. ;-)