Mustache-Moustache | Commonly Confused Words

Mustache

/məˈstɑːʃ/

Facial hair grown above the upper lip.

This word has no antonyms

/məˈstɑːʃ/

A strip of hair grown above the upper lip, typically by men.

This word has no antonyms

Mustache is the spelling used primarily in American English, while moustache is the preferred spelling in British English and other variations like Canadian or Australian English.
Both words refer to the same type of facial hair grown on the upper lip.
The key difference is just the regional spelling preference, not the meaning.
Remember, US is in mustache, which can help recall the American usage.
The extra o in moustache could be thought of as an emphasis on the pronunciation, used outside the US.