For a college/university, they would be a fellow alumnus (or alumna).
Thank you very much, SoothingDave.
I thought of using ‘alumni’. But it was my secondary classmate posting our recent gathering photos on facebook. One of our classmate he lives in the U.S. now. He saw the photo and said I looked familiar.
Then I thought how to introduce myself to him. It is similar to a situation if I meet him one day.