Mouth dryness when using a nasal mask is usually caused by air escaping through an open mouth. This can helped by using a chin strap, or if this is not effective, switching to a full face mask (which covers the nose and mouth).
In patients using a full face mask, some mouth dryness can also be felt especially at higher CPAP levels. This can be alleviated by use or increase in the heated humidification, and ensuring that there is not excessive mask leak.
In addition to these equipment changes, you may want to use oral tablets of gel that keep your mouth moist throughout the night. Two such products that patients swear by are Xylimelt tablets and Biotene gel. Both of which are slow-release saliva stimulants and oral lubricants that provide hours of relief from dry mouth. These can be found at your local Walgreens, CVS or other well stocked pharmacy.