It could be some slight fin damage, or if it is a new fish, it could be Lymphocystis. More background information and a picture under whiter light would be helpful. Here is a link to the background info that we need:
Aquarium Fish Disease Problem Solving Determining the best course of action for a problem with your aquarium can be a difficult task. Even advanced aquarists will often need to seek the help of others for complicated or unique issues. Our intent here on Reef 2 Reef is to offer expert advice...