It's not just his face u know, yeomin is very bubbly and giggly in front of him and really doesn't say no to anything he says- making him appear like an easy target. (Which is what taeseo thot of him too). So the fact that he used that face and tone, knew how to set boundaries saying "Why would u think I would buy stuff for u, I'm ur teacher" is the big deal. He realised that, he doesn't really let just anyone into his circle.
Obviously - he didn't hear the words told but he understood what was happening
Ahhhhh okay that makes sense
I’m sorry but the face that yeomin made literally looks no different from his usual face i don’t know why taeseo was so shocked