Judging by this woman's features, Nensu had to assume that she was Izuku's mother, and by her response to what the teenaged girl called her, she was mentally right. Before she could even acknowledge or answer the question, the boy had already left the talk room...until the woman practically crashed into him, clearly very worried and scared for the wellbeing of her son. 
The girl didn't know what else to say, considering that there was rather a very awkward tension in the air, considering that Mrs. Midoriya probably had no idea that she and her son were even dating, before Shigaraki showed up. Nensu stood up from the chair, and lightly hugged herself. 
What was she supposed to say to help ease the feeling? The last thing she wanted, was the worry the poor woman even more.