What do you call a fifth dan and a sixth dan black belt?

In Korean terminology, 5th degree is Oh dan, and 6th degree is Yuk dan, and that's it. In Japanese they would be Godan and Rokodan.

Any titles in addition to that must be earned separately. There are no titles that are automatically assigned to a rank. Titles of Instructor, Master, Grandmaster, etc. are independent of rank, although many organizations require a minimum of specific rank level to be eligible for such titles. The rank eligibility for such titles vary between organizations, and are awarded at the discretion of the director or Grandmaster (Kwanjangnim in Korean).