Maybe your pleco is just a weirdo like some loaches are. It's an uneasy feeling to suddenly see a loach laying on its side all pale & dead looking only to find out that it's common for them to relax in that manner. I can't say that it's common behavior for a pleco to do what yours did though. Raph has a good point. Check your fish all over really close up. He might have the beginnings of some good ol' fashioned ick. If so, it won't be very long from now until it becomes blaringly noticeable, but best to detect it early so you can treat it early.