According to what sounds like one of the caretakers in this video, https://www.youtube.com/watch?v=TVOjmX1J70Y, his name is Juno (it’s a ‘he’) and it is an aggressive, territorial display of dominance called “jaw popping”.
ETA:
Got to be a completely unbiased account if even the handlers are calling it aggression. If the handler was saying “oh, he just loves to play with all the guests”, then I’d say we had competing views from two biased sources. But it looks like the experts on both sides of the “whale captivity” debate agree that it is aggressive behavior.