That shake of his head is all that Terezi really needs. If Jack can't bring himself to kill her, then there's no reason to hide from her, either. She's not a threat, as far as Terezi's concerned.

"That might just be the saddest thing I've ever smelled in my life." She sits down on the branch, her legs swinging idly but still out of reach. It's obviously that she doesn't feel threatened in the least. "What are you going to do? Close your eyes and swing at us like some kind of wriggler piñata game? Seriously, put the branch down before you hurt something other than your dignity. Hehe."