What species is this? My clown pleco has never looked like this, the yellow stripes are still a bit darker then this, although maybe it's the difference in lighting? Is there any fish that could be harrasing the pleco in your tank? What are your parameters temp etc? Is there caves and cover for it to retreat to? I'm sure someone will know a lot more then me on this topic, but I've seen a black veil angel turn almost white when it's bag leaked during transport before, so it could just be down to stress.