Zayn Malik was in a MOOD Saturday when he touched down at LAX ... maybe he's missing his lady love Gigi?

Or maybe he's disappointed that his lady love Gigi doesn't even know what ethnicity he is! In a recent interview, Gigi -- whose dad Mohammed is Palestinian-American -- told The Sun she feels connected to boyfriend because he's half Middle Eastern. Except he's not ... his father is British Pakistani and his mother is Irish. And guess what, folks -- Pakistan is not in the Middle East, it's Southeast Asia.

The Twittersphere went wild slamming Gigi, one "fan" writing: "Gigi Hadid said Zayn was half Middle Eastern. GURL PAKISTAN IS A SOUTH ASAIN COUNTRY. LEARN YA GEOGRAPHY." Another added: "Just to let you know Zayn is Pakistani, which is in South Asia. Just in case you didn’t read his profile properly before signing."

So maybe that's why Zayn looks so moody?