That's most likely stress. Many fish, plecos especially, changes color when stressed. Usually will see lighter patches appear, in some cases, entire fish will be lighter in color. In fact, that's how many online vendors sell their fish. They take pictures of stressed out fish, which are a lot lighter in color, thus appearing brighter. You see that very often in L25's base color or L27 goldlines that shows bright lines. Also will see L27 Xingu platinum that shows a white base, only to turn dark grey later. Many get disappointed when the fish turns darker and more "normal" as they settle down in the tank.
Stress can be just from transferring the fish, new environment, bad water quality, bad tempered tank mates, or disease. You'll have to figure out where the stress comes from.