"That girl" is the TikTok aesthetic of disciplined, wellness-focused young women: 6 AM workouts, journaling, matcha, neat apartments. Not concerning, mostly positive habits, but can fuel comparison-driven anxiety. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.