The US Marines greeting is "Semper Fidelis"(Semper Fi,shortened form).
May a member of another branch of the armed forces(i.e.,Air Force)use
this greeting/farewell when adressing a marine or is motto only to be
used execlusively between marines?

I have tar friends who would be ready to brawl with a leatherneck who
greeted them with "Semper Fi."

The zoomies I know would just be confused.

And anyone in the Army would be so embarrassed about their slogan, "an
army of one" that they'd go hide and completely forget their motto.

Oops, forgot the most important bit. Every Jarhead I know would be
tickled to be greeted with "Semper Fi, Marine!"

But cut a wide swath if they've been drinking.