That isn't really a sign of anything specific. Could be flukes, could be bacterial. There are some rarer issues like an internal fungus disease. It could also just be excess mucus due to some irritation.
Past history of the fish would be important to know. Here is a link to some of the various types of information 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...
www.reef2reef.com
Most important would be - how long have you had the fish, what has it been treated with, any other symptoms?
Jay