It looks like Velvet to me honestly, all the dense goldish dustings. I'm actually treating for velvet now, it can wipe out a tank if not treated properly. Coppersafe is the recommended med but not sure you can use it with the Pleco; also if you have snails or any other inverts you can't use it. I have COrys so I couldn't use Coppersafe either.
Here's what I'm doing to treat velvet:
First, remove any inverts you have to another tank or holding container, etc.
1) Do a large (70-80%) water change with gravel/sand vac. Refill with dechlorinated water.
2) Get some Maracide, PetSmart should carry it. It comes in small bottles though so you might need a few bottles at least to get through treatment (I have a 40 gal and go through 3-4 bottles in a week). Remove any snails and/or shrimp you have; put them in a separate small tank if you can with a heater and an air stone.
Dose as it says on the bottle (2 caps per ten gallons on days 1, 3 and 5).
3) Slowly raise the heat to 83 over a couple of days. This will quicken the life cycle. If you have an air stone, turn it up. The higher temps will cause less oxygen in the water for the fish.
4) Cover the tank with blankets so light does not get into the tank. Do not turn on the tank light and cover the tank to prevent ambient light coming in. Velvet needs light to multiply.
You can turn on the lights for a few minutes each day just to feed and check for dead fish (remove any dead fish ASAP) but that's it.
On day 6, do another large water change and vacuum the substrate thoroughly and start the process again. Do this for at least 2 weeks. Hopefully the treatment will work. You can use other meds in conjunction, like Maracyn, but meds like that often will affect your good bacteria in the tank so don't add anything else unless you really need to.
I'm on my 2nd week of this treatment and it seems to be working, but I think I caught it early (I only saw the dustings on 3 fish). Good luck and keep us posted.