Cotton like spot on my Coral Beauty

[Nøjo]

I've had the fish for only about a week and have just found a small white spot on his spinal fin, just before his tail. While it is not much bigger than a pinprick it did get bigger overnight. It looks like a tiny ball of cotton stuck to his backside. Any ideas on identifying or treating the problem?

 

[steve-s]

Could be Lympho or bacterial. Is the fish in the display tank or the QT? Can you post a clear pic of the area?

Cheers
Steve