Lang Su would have been able to stay longer if she had not used her powers to attack, but Min Qing's body was too weak to handle a fraction of her powers.
Furthermore, if she should reduce her power output, she knew fully well that she would not be a match for Little Wu alone, not to talk of the rest of them, so she had to return.
After all, she wouldn't want to injure her perfect vessel.
Thus, the moment she spotted a slight opening, she dashed towards Min Qing and straight into the seal within her forehead as she left Min Hong a message;
"You're so shameless, how could you touch and grab a little girl such as myself in such an inappropriate manner."
"We will be seeing each other very soon, and I will be sure to pass your shameless acts to my father before then…"