These guys would have for sure known each other, Moon likes to keep himself pretty firmly attached to Joon's hip a lot of the time. He would be very happy to have Hyojin treat him like that, might be the only older brother type he'd probably be used to and comfortable around as well. He's excessively shy and scares easily around people, even if it just comes off with him looking extra grump/emo/or sassy a lot of the time.