To my knowledge, Willie Mays is the only player to hit his first home run off of a Hall of Fame Pitcher.
On May 28, 1951 Mays, who was 0-12 in his Rookie Season, took Warren Spahn deep for 450 feet.
It would be the first of 18 HRs Mays hit off Spahn--the most off any single pitcher in his career.
Mays mentions his slow start in this video: Leo Durocher encouraged Mays who was feeling down after being hitless in his first 3 games.
After Durocher's encouragement, Mays hit his first HR the next day.
https://youtu.be/2SixtuO41L0
By the way, Duke Snider has the record for most HRs off a single pitcher (Robin Roberts) in his career with 19. Musial also had success against Spahn, hitting 17.