why was everyone calling him a Groomer?

there was no romantic or exploitative intent or behavior toward the person while they were still a child.

grooming involves building trust with a minor for the purpose of later sexual or romantic exploitation.

though they had a relationship when yooh-an is now an adult and Jaehee knew him when he was a child there we’re no grooming involved?

Jaehee was inlove with Yooh-an’s father so when he called him Mom, jaehee liked it because he likes the idea of being the mom meaning he is the WIFE of yooh-an’s father