Morty Smith

When watching the film Back to the Future with Bill Potts, the Twelfth Doctor misidentified Marty McFly as "Morty". Upon correcting him, Bill stated that "Morty" was "a whole other thing". (COMIC: The Great Shopping Bill)

Behind the scenes
The that Bill is referring to is the character from the adult animated comedy. The titular characters were originally conceived as parody versions of Back to the Future's main characters, Doc Brown and Marty McFly, hence Bill's response to the Doctor misidentifying him.