Is the oxygen content from your mask actually higher than normal air? Even if it is, you won't get spontaneous combustion. You need a spark for that, or very high heat. Any wax you like should be good but a lot of waxes have petrolatum in them which can harm natural rubber or latex and damage a mask if it's made from that.
Have you tried Aleene's Clear Gel Tacky Glue? It's greaseless, holds stronger than wax and washes out easily. Check this article and video on my blog.http://handlebarmoustachelife.com/moustache-grooming/video-style-your-handlebar-mustache-with-aleenes-glue/