Welcome to AC!
Well, I would be concerned but not panicked yet. Tell us all about your tank & pleco. What size are both tank & fish? How long have you had them? How often do you change water & how much each time? Do you have a test kit & what does it say? Do you have other fish? What & how big?
It may have scraped itself along those rocks looking to hide...or it could be the beginnings of a bacteria or fungal infection. How long has this splotchiness been going on?
Sorry, many more questions than answers yet...& we may have more.
For now I'd do large water changes as often as you can (daily? every other day?), they can work miracles! Or at least keep things stable until you & we have a better idea of what to do next.
His nose looks weird to me, (bumpy?) maybe just the pic angle? Try for a few more shots, head & splotches, as close as you can focus clearly. That may help us help you.
