Ghost Yoda is the teacher of Ghost Anakin. When teaching him, Anakin's power was very strong. This was 21 BGA - 8BGAhe found a giant cliff near Mount Odlid. It imploded and his walking stick broke and he fell down and died.
Ghost Anakin was devastated and used his newfound powers to make his mentor a ghost. Then Yoda came and joined Ghost Anakin's seating council.