He's surprised when Roger tells him that he and Mimi are over. As far as Mark knows everything's been okay between them. No fights. No relapses. No insecurities leading to infidelities. And Roger is calm, which, when it comes to Mimi, never happens. It's always blind joy or blind rage. Not in between.
It's just weird, and when he asks why, all he gets from Roger is, "We decided it wasn't working out, and figured we should just move on."
"Move on to what?" Roger is about to say something, but stops himself. "Did something happen?" It's a stupid question. There's a hundred reasons for them to move away, from each other, at least. It could be Mimi's addiction or her relationship with Benny or Roger's need to save someone after he couldn't help April - things they just can't get past, though they try. But moving on means moving to something else.
Then again, he's hopeful. He doesn't want to pry, because he's afraid it might slip that there's a part of him that's relieved that this turned out the way it did. So he lets it drop, and doesn't notice when Roger tries to speak but doesn't seem to be able to find the right words. He doesn't notice that the words Roger is rehearsing are not lyrics, but parts of a confession.