A "melasma mustache" is essentially just hyperpigmentation gathered over the upper lip, hence the name. Some may experience melasma in other regions of the face, like the forehead and the cheeks, but the upper-lip area is super common.
"Although melasma can occur in men, it's much more common in women and more common in those taking oral contraceptive pills and during pregnancy," board-certified dermatologist Rebecca Marcus, M.D., tells mbg.
Clinical studies show that UV radiation is the most common initial trigger of melasma, as well as the main factor exacerbating preexisting patches. "Even a tiny bit of sun can cause melasma to flare in those who are susceptible, and even heat, in the absence of sun, has been known to cause a flare," Marcus says.
Hormones are also a common cause of this kind of hyperpigmentation (which is why it's commonly triggered by birth control pills and pregnancy), though it's unclear if the upper-lip area is more frequently affected by one of these triggers or the other.
However, if you are only experiencing hyperpigmentation above the lip, you may want to look into your skin care practices as well—hair removal methods included. "I believe that melasma flares in the upper-lip area are frequently triggered by hair removal. Whether laser (heat plus light) or mechanical trauma from waxing or threading, certain hair removal modalities can trigger inflammation that may lead to a melasma flare-up," Marcus explains.
So while the exact reason for the melasma mustache is still slightly unknown (especially if other patches of hyperpigmentation aren't present), hair removal may be something to consider.