User blog comment:Kaijusaurus-Rex/Season Five Info and Leaks/@comment-28485972-20170205051543/@comment-28316467-20170208235610

According to the source:

"There will be some kind of a The Amazing World of Gumball-meets-Frozen episode, but it will be more of a satire of the princess myth than a cross-over."

So it's the animated Frozen movie.