RUBATO. Yes! That's the right term to use to describe what Bix seems to be doing here. It is, of course, one way to improvise on a melody, with or without altering the notes of the melody, but using that term better nails down our differences in interpretation of what was going on at that point. He's not playing "bum" notes, discordant notes, but he's taking some liberty with the melody and also with the rhythm of the melody.