My pleco seems to switch colors every handful of days. She'll be completely dark and uniform for a week, then for a couple days she'll get these lighter patches before eventually going back. She seems completely fine, just switches up colors every now and then. My example of her when completely dark is from when she was a bit younger by the way, but the one of her with lighter patches is from a few days ago. Its also aways the same spots, there aren't random lighter patches all over her body, but the same areas.