Retired Marines do use the phrase "Semper Fi." Short for "Semper Fidelis," the U.S. Marine Corps motto translates from Latin as "Always Faithful." The term is used both in direct reference to the official motto and as a colloquialism among Marines.
In a video on the subject from the Marine Corps website, one Marine says, "There's no such thing as an ex-Marine, no such thing as a former Marine. Once a Marine, always a Marine." Active and retired Marines alike adhere to the principle inherent in the phrase itself and see their loyalty as a lifetime commitment. They may use the words "Semper Fi" as a way of addressing other Marines.