Fauni and Uno actually stole this beat from GRXGVR without crediting him. (They probably learned this behavior from their frequent collaborator Playboi Carti, who has a history of stealing beats) The beat is a song called VARGTIMMEN from GRXGVRs 2014 EP “♰ 4EVER YOUNG ♰”:

In the comment section of this soundcloud track, GRXGVR acknowledges that it was used without his permission:

“Xanaxx” is a made up name to distract from the fact that the beat was stolen. “Xanaxx” is not an actual producer.