Still trying to figure out if Mr. Fantasy is actually KJ Apa ? Yeah ... good luck with that -- 'cause when we asked him straight up, the guy got real weird. TMZ caught up with the eccentric Brit in Hollywood Tuesday ... watch the video, as things quickly took a bizarre turn when we pressed him on the rumor he's the "Riverdale" star. Let's just let the video do the talking ... We tried our best -- even asking how often people mistake him for KJ -- but he kept insisting he had no idea what we