I checked, Mary-san is a Japanese urban legend, basically a person gets a phone call from a doll named Mary, and she slowly gets closer and closer until she's behind you, then if the person turns around they'll get stabbed to death by said doll.
Something like that.
And Yuka didn't even call before showing up, so Hajime must have been referencing the urban legend.
Also Hajime seems to know a lot about ghosts and spirits for some reason.