new Dreamware nasal mask user

Using Jason's advice from one of his videos, switched from a full face mask to Dreamware nasal mask. First use, very comfortable, no leaks. The pressure is set at 15 from full face setting, the air pressure blows my mouth open, will lowering the pressure help with this problem?

Lowering the pressure most likely won't help with the air entering the mouth via the airway and causing the mouth to open.
This is a common problem and isn't limited to higher pressures. I know people who have that problem happen with as little as 6 cm pressure.
Plus if you reduce the pressure there is a good chance that it won't be adequate to hold the airway open and prevent the collapse or apnea events from occurring.

The sort of semi official term for your problem is called "chipmunk cheeks" and you can search the forum for the numerous past discussions about it and some ideas how to reduce the chances of it happening.

You have to close off the back door of your oral cavity so that air going up the nose and down the airway doesn't/can't try to exit prematurely by sneaking into the oral cavity causing cheeks to inflate and mouth to open. Easy said than done for some people. I can be done though. I can open my mouth and talk and stick out my tongue and blow raspberries while wearing the mask with the machine on...and not one tiny bit of air enters my mouth. I am not the only one but it did take me some time to get to that point.