Hi and welcome to the forum!
I believe the catfish (nice fish by the way, I love raphaels!) has a fungal infection. There are multiple ways to treat it, however the most effective are an antifungal medication or salt. Method of dosing salt is below and safe for all aquarium inhabitants and is plagiarized by one of our more informed members (thanks
@Colin_T).
"You can add rock salt (often sold as aquarium salt), swimming pool salt, or any non iodised salt (sodium chloride) to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres (5 gallons) of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for 1-2 weeks.
The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
When you first add salt, add the salt to a small bucket of tank water and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes."
If you have any anti-fungal medication, that'll defineately help.
Also, just so we can further help-
What are/is your...
Exact water parameters
Other aquarium inhabitants (do they have any other symptoms? Actually, does your catfish have any other symptoms?)
Tank size
etc.
If you have a quarantine tank, I would suggest putting it in there to avoid spreading it to other fish. Fungal treatments like this can *usually* be prevented by keeping good water quality.
Good luck and hope this helps!
PPJ
EDIT- Also, why put it in this section? It seems kind of random. The mods will probably move it to 'tropical emergencies'. No judgement, was just curious