At first this happens because you are concentrating so much on your hands that the rest of your body can move in sympathetic ways - not just gurning faces, but you find some guitarists twist their bodies, arching their backs - not always deliberately :-)
Once you have learned the 'muscle memory' to a level that playing is automatic, the gurning reduces and
you become much more relaxed.
If you are pushing the limits of your capabilities it may happen.
Personally, I wouldn't worry about it - once you can play well, you'll be forgiven a lot of things.
music.stackexchange.com/users/104/Doktor Mayhem
The jazz nod and the blues grimace are things that many players practice for hours in front of a mirror to ACHIEVE...and you want to practice to ELIMINATE them?
heh
Try singing along.
You can't make weird shapes with your lips if you're using them at the same time as you're playing.
music.stackexchange.com/users/21900/dwoz
Circuits in your brain controlling your hands while playing, those circuits in some people are right adjacent to the circuits that control facial muscles.
There can be a little bleed through, like faintly hearing the wrong phone call on your land line.
If you force yourself to smile while you play it may help.
music.stackexchange.com/users/31390/JEDD
Only eliminate a guitar face if it makes you look ugly.
Consult the opposite sex on this.
If it concerns you, practice playing in front of a mirror.
Also practice in front of a mirror moving from your guitar face to a relaxed face.
You will learn how it feels when you make the face and have the muscle memory to return your face to a relaxed position.
music.stackexchange.com/users/10635/pro
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. ;-)
music.stackexchange.com/users/27335/General Nuisance
I chew on my tongue when playing guitar.
I am playing guitar for 30 years.
But because I hardly ever perform I am not aware of my facial expression.
It looks as if I am chewing a gum bit
then I stop and you can see that I am not chewing any gum.
It looks weird.
A musician friend suggested I actually should chew a gum to prevent that becasue it is too late to eliminate that habit.
So instead of hiding your mimic you expose it.
music.stackexchange.com/users/44949/vid
When I'm playing (learning) difficult chords on guitar (ex: F,B7) my facial muscles (basically lips) form a weird action (I felt it and I saw it in the mirror).  I feel I'll get used to it as I go and I have seen some good players also do that (may be they can't help it).  You may also have faced this experience in learning stage and got over it. How did you overcome this?
How can I avoid making unpleasant facial expressions when playing difficult chords?
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<guitar><practice><performing>
music.stackexchange.com/questions/6784
music.stackexchange.com/users/1391/DinushanM
There are a few ways to stop making strange facial expressions when playing the guitar. You can sing along to the song, play in front of a mirror or film yourself while playing. If you want to avoid making unpleasant facial expressions, you could try practising in front of a mirror or singing along with the music at the same time as playing.
Try singing along to prevent unintended facial expressions. You can always try practising in front of a mirror to try and eliminate unpleasant facial expressions.