The spot looks too large to be a typical parasite. It is either fin damage from another fish or Lymphocystis, a viral disease that may get worse, but will go away on its own after a month.
How long have you had this fish? The picture isn’t clear, but it looks kind of thin to me. How is it eating and what are you feeding it?
Rock beauties are one of the more delicate angelfish. They seem to eat ok at first, but often waste away and die. I had one I kept for two years, but it never really grew in size. They probably eat mostly sponges in the wild.