'Extant' Star Sergio Harford -- If I Met Malia Obama ... I Didn't Know It!

Malia Obama blended in during her stint as a P.A. on the set of "Extant" so much ... one of the stars of the show had no clue he even met her!

Our photog caught up with Sergio Harford in Hollywood Monday night ... where he said people have told him he talked to Malia while she worked on the show over the summer ... but he has no recollection of it whatsoever.

He also claims the Secret Service -- who surely HAD to be around there somewhere -- were nowhere to be seen ... which makes us think -- maybe they were extras.