This looks like a classic case of ulcer disease to me. It could be the pleco, and I agree with Kashta that it would be best to move the pleco out of the tank. If not because it might be bothering the fish, but more because any meds you add to the tank can sicken or kill the pleco. They are very sensitive fish and ulcers require powerful medications. So you will need a rubbermaid tub or another tank to move the pleco to.
First you're going to need to deal with the ulcer itself. This is a very, very easy procedure. You can get clove oil online or at health food/aromatherapy type stores.
You'll need a plastic tub, an air pump, clove oil, q-tips and neosporin or bio-bandage. Don't use just any cream or antibiotic gel. The dosage for humans and fish can be very different. Neosporin is the only human antibacterial safe for use on fish.
Add the clove oil to a gallon-size ziplock bag of dechlorinated water at 5 drops per gallon of water and shake like crazy until it is fully dissolved. Add that to the tub and repeat until your tub is full. Add the air pump to keep the clove oil in solution.
Net your fish, put him in the tub and wait a minute or two until he floats onto his side. Be sure his gills are still moving. If they stop, add more fresh water to the tub to dilute the anaesthetic.
Lift him with a wet towel until his ulcer is exposed, carefully pat it dry with a PAPER towel, dab on the neosporin and then lift him back into the net. Hold that in the main tank until he wakes up. You can slowly swish him back and forth in the water to wake him up quicker.
Once you have dealt with the ulcer itself, you're going to need a powerful antibacterial medication. I recommend a Maracyn/Maracyn II regimen. You might want to put him in a 10 or 20 gallon quarantine tank while you treat him so you don't need to buy a huge amount of medication.
Best of luck with him.