A mustache will generally prevent you from gaining a watertight seal with your mask. The mustache breaks the natural seal created by the mask's silicone skirt and bare skin and people who have a mustache will generally complain about water leaking into their mask constantly. Depending on how bushy the mustache is will dictate how bad the leak may be.
Snorkel Mart identifies 3 possible solutions to problems associated with snorkeling with a mustache. These solutions can be combined or observed individually.
Shaving the mustache slightly just under the nose will often expose enough bare skin for the mask to create a seal.
Apply some sort of gel to the mustache. We recommend using silicone grease.
Installing a purge valve in the mask will provide an easy means of expelling any water that might leak into the mask at the mustache.