If your Pleco is eating and acting normally, not glancing off rocks or shimmying, chances are he is ok. How are the other Fish? Is the Pleco a new addition?
If it does turn out to be ick there are numerous medications available with varying instructions. I prefer the medication free method.
-raise water temp. Up to 86 degrees
-15% to 20% WC (gravel vac) every other day for 10 days.
-some members add a little aquarium salt as well.
I'm certain that other AA members more experienced with ick meds. Will offer advice soon.
It's been my experience that Plecos and catfish are very disease resistant and are usually last to get I'll. Good luck.