Ted looks down at her, surprised she would notice his tension enough to ask. The fact that she does makes him feel kind of cornered - even if he knows that's not her intent.

But maybe she doesn't realise why he suddenly got tense? After all, he's got plenty to deal with right now, she could just think he's stressed out thinking about Rupert. (Which absolutely has been making him feel tense, and now that he's thinking about that too, the tension seems to redouble.)

He takes a breath and intentionally relaxes his shoulders, and shakes his head a bit. And he also reaches out to take Joan's hand, squeezing it a bit.