Guide tea is warm
Gu Beiqing sings cold songs.
So I guess there is a possibility:
1, your ex-boyfriend wants to use the couple's screen name with you;
2, jump off the couple's online name or something, he thinks this sentence is very suitable for him, and his heart is lonely;
3. Further, he wants to express his love euphemistically, and send you "Gubei QingHan Ge", expecting you to spend time with your mother and then have a look.
"Know the warmth of southern tea" and then express the meaning of "I think you are a warm and loving sister".
These are all my brain holes for reference only.