He has noticed Serafina coming and had intended on greeting her, but her focused eyes didn't even find him. In fact, if Jojo hasn't stepped to the left at the last minute, they would have collided. Laughing, he said her name twice to get her attention.
"Sera? Hey, Sera! Where are you off to in such a hurry?" Joseph called after her. He u-turned to trail after her, sort of surprised she wasn't with her Slytherin friends.