I've seen those bands on many a "common pleco" - they come and go. I'm sure someone knows more about this, so hopefully they'll chime in. they could be related to mood, territoriality, camouflage (really - most "plecos" come from fast moving rivers so this could look like the ripples of sunlight on a river bottom), or something else entirely.
Don't worry, your fish will not "lose all his spots" or get whiter and whiter. Come to think of it, it's been a long time since I had a pleco - I don't recall seeing those bands when they were really young, but they might have started showing up at a certain size.
so the long and short of it is - nothing to worry about - but I'd be interested to know what function ichthyologists think they serve.