Robbie Williams is feeling minty fresh ... because he says that's what fell on his mic live on air in a viral moment from the World Cup final ... and not nose candy, as some speculated.

Here's the deal ... the UK superstar was doing an interview with German TV during the big game between Spain and Argentina when a speck of white substance flew from his face onto the mic. He picked it off, held it in his hand and then rubbed it on his forehead.

Folks online claim the white substance came falling out of his nose ... but a closer looks reveals the chunk came out of his mouth.

Robbie posted a video Monday leaning into the cocaine speculation ... and made it sound like he was about to make a deep confession -- before laughing it off and showing a couple mints on his tongue.

He captioned his video response ... "Just woke up and feeling…. Mint ???? x."

Robbie was a part of the massive halftime show that featured Madonna, Justin Bieber, and Shakira.

Bottom line ... Robbie's no Diego Maradona.