I've always sported a Mustache. I have done some snorkeling over the years and never had any problem clearing my mask.
But during our pool sessions I have noticed that I tend to constantly blow % air out my nose to keep pressure in my mask. Last class we had hoods on and the air escaping the corners of my mask was bubbling up and under the edge of my hood creating a bubble on top of my head!

My concern is that as I dive deeper mask squeeze will become an issue without the positive seal around my upper lip. My instructor has assured me stating that depth will actually help to create a more positive seal and prevent mask squeeze. I know (& see...Zero) others who sport 'staches' who are divers so, it has to be a non issue, right?

Dave