I concur, I think this is what it is. Unless the patches are fuzz... are they?It could be your light coloured substrate as it might try to blend into it's environment, my female did the same in her first week and her colours have come back slowly, my substrate is light in colour too. I do also suggest wood for it's diet.
I also agree that plecos need driftwood. They will suck on it and get wood fiber, which helps with digestion.