Do not italicize in regular usage. Capitalize only when referring to a proper name.
Thank you for supporting your alma mater.
The Citadel Alma Mater (name of song).
alumni
Use the word “alum” to denote a single graduate, male or female. “Alumnus” refers to a single male graduate, “alumna,” to a single female graduate. The word “alumni” is plural and should only be used to refer to a group of male graduates or mixed group of graduates. The plural for female graduates is “alumnae.”