Hi and welcome to the forum
The fish has excess mucous over its body. This is caused by something in the water irritating the fish.
Check the ammonia, nitrite, nitrate and pH of the water?
Do a 75% water change and gravel clean the substrate every day for a week or until we work out why it's producing the excess mucous.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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How long has the tank been set up for?
How long have you had the fish?
How long has the fish been patchy?
Have you added anything to the tank in the 2 weeks before this started?
How often do you do water changes?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
Do you have buckets specifically for the fish tank?
What sort of filter is on the tank?
How often and how do you clean the filter?