Thanks! Why does Yamachan get called Hosei now?
He studied to be a rakugo (comic storyteller) between 2008 - 2013 and after graduating he took on the title of his teacher, Tsukitei Happo, as a professional family name. So officially he's called Tsukitei Housei, but people tend to call him Housei, I'm not altogether certain why, I just guess it's because there's a few different Tsukitei's in the industry (e.g. Happo's son and fellow rakugo, Tsukitei Hachimitsu).
Due to their familiarity he lets Downtown still habitually call him Yama-chan / Yamasaki, but to everyone else he's Housei or Tsukitei Housei.