A two shot, in which Haechan is suffering from Mark's sudden distance and doesn't think he can hold on any longer.
So, he does the thing he is best at: singing. Singing it out. He needs to say it out loud to move on, even if he didn't say it to Mark directly - he knew he couldn't, that he mustn't. Their friendship, or what's left of it... he needed to keep it, even if it was just a perfect illusion. To him, living in an illusion was better than living in the truth. But this one, just this one, he'd face the truth.
So he does the thing he is best at: singing... in front of hundred thousands of people.