Moisture in the air alone won't help. You have to find a way to breath through the nose instead of mouth breathing. Some people use chinstraps to keep their mouth from opening involuntarily and some even use special mouth tapes. I wouldn't recommend mouth tape but a chinstrap may be worth a try

Reading, for the three separate problems this thread braids together: CPAP Dry Mouth and Mouth Breathing: How to Stop It Tonight — the dryness itself. CPAP Mask Leaks & Mouth Leak: Causes, the Data Threshold, and Fixes — the leak checklist, in order. CPAP Nasal Congestion and Stuffy Nose: Causes and Fixes — for when the nose is part of the story. Nothing on SomniTalk is medical advice. Talk to your own clinician before changing therapy.