I think this time around, Haejoon himself is unaware of the ghost. And the ghost seems to have two effects on Haejoon - he falls asleep pretty much immediately when he's around her, and she makes him hyperfocused about tutoring Eunyung. Guess she's possessing him, and she was a tutor in her life?
Haejoon does see the ghost, his just pretending to ignore it. I'm like 99% sure this is Miyeong, thos ghost whose been watching over him since he was a baby. Which is why I think it's so easy for Haejoon to not freak out around her, because she's a ghost that brings him great comfort.
In an earlier chapter she tried to follow him out, but couldn't leave for whatever reason, and we also know this was Haejoon's old room. I think the reason why Miyeong can't leave, and why Eunyung can see her probably have something to do with the talasman.
IMO there really isn't any reason to believe this isn't Miyeong, since no other ghost are present. Ghosts only appear in the building when there's less than three people living there, and the ghost looks exactly like Miyeong, who has been a constant presence in the story. I think Haejoon is just being stubborn, and doing things his own way like he usually does, not really considering Eunyung's feelings. Because Haejoon
knows Miyeong isn't bad, but doesn't seem to get that Eunyung couldn't know that and would rightfully be freaked out over her.