Emet-Selch wants to laugh, wants to ask Hythlodaeus what he knows about how people look at him, since he clearly seems to be oblivious about it. But there's something else about this whole encounter that prevents him from doing any of that. It's something that only grows stronger when Hythlodaeus so dangerously grasps his chin.

Forget? Forget. If only there were things that he could forget. This would not be one of them.

"And if I do?"

He chooses to antagonize this side of Hythlodaeus instead, just to see what happens next.