It looks like excess mucous caused by poor water quality or something in the water irritating the fish.
The edge of the pectoral (side) fin is probably mucous too but might be fungus.
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What is the ammonia, nitrite, nitrate and pH of the tank water?
How long has the tank been set up for?
How often do you feed the fish?
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you do water changes and how much do you change?
Do you dechlorinate the new water before adding it to the tank?
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Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will dilute any nutrients and reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
