A picture will give us an Idea of what this looks like.
It sounds like ich but also fin rot but another fungal infection.
Have you treated ich remembering to take out the carbon from your filter media?
Some people say use aquarium salt for any disease but you should look it up and see just how much to use.
If your water is constantly cloudy and water quality is of a low standard, this invites those parasites to have a party. Try setting the heater up a few degrees.
Like I said, it sounds like a lot of things. If your fish is fading then it has an immune illness and is bound to die without proper care.
Treat the water for ich again but turn up the heat and remove your carbon from the filter.
If the fish stops eating, isolate it in a hospital tank with a slow sponge filter and treat for ick at half a dose for the first 24hrs but later add the other half dose. This lowers the stress levels caused by your treatment.
Be sure to read the instructions on the chemical carefully before you treat your fish. Some fine scaled species require a lower dose than catfish. This wont harm your pleco.
Don't do too many water changes. Try to keep the water in the tank for about 5 days. Change the water and treat it again if the spots are still visible.
Feed them some live or frozen food after and during the treatment. This stimulates blood circulation in the fish which speeds up the healing of the fin rot issue.
These are just some things to take into consideration. Many people take feeding during illness for granted. Flake food wont get your fish through the illness.
Hope your tank is healthy again.
The other "stuff" i am talking about with regards to what you described is Body Fungus or a columnaris infection.
Columnaris looks like this