The call?

[Rick KY]

You should have nothing but a warning for the offending player, since you need to see if the mouthpiece was hanging loose or in his mouth at the snap. If it was in and came out during the play there is no foul.

If you see the thing hanging loose you should warn the player to put it in his mouth before the snap to prevent a foul. Officials should try very hard to handle this situation using preventive officiating, this foul call should be discouraged. I'm not saying ignore it. If you throw a flag before the snap, you've got delay of game. After the snap you've got failure to wear required equipment during a down.

Handle it before the snap and you avoid a foul, or limit the penalty to 5 yards.
After the snap it's a 15 yarder.