For me, it just looks like he has a certain coldness in his eyes. It's not a dead or vacant look, it's just the way a smile, or any other facial expression for that matter, just doesn't seem to make it to his eyes. There's obviously life and intelligence there, but it's not a friendly intelligence. I pulled up the most lizard-man pictures of Zuckerberg for comparison, and even at his most robotic, his eyes still look human. Like there's some capacity for empathy in there somewhere. With Musk? His eyes just don't quite read as human to me in an uncanny valley sort of way.