I would guess lymph. Caused by stress typically, can also be caused by poor water quality. There isn't a surefire treatment. Its suggested to feed vitamin enriched food and reduce/eliminate the stressor. It can take months to go away. I have one disease book that says visine has been shown to help in extreme cases. I will have to look at the book to figure out if they give any guidance on using visine. I had a Emperor angelfish years ago that had it and it took almost 3 months to go away