Dont worry its natural.
A bit about feeding:
Plecos are omnivores. They are not picky eaters. In planted aquariums they will eat any food left behind by other fish as well as naturally growing algae in the tank. They have also been known to have a taste for discus mucus. In tanks without live plants, their diet can be supplemented with sinking fish food, usually algae wafers readily available from a pet store.
Although the plecostomus is a known common algae sucker, it requires more than just pellets and tank algae to eat. It should also be fed algae wafers, zucchini, cucumber, lettuces, peas, melon and also any semi-soft fruit or vegetable will do as well. They also can eat shrimp, shrimp pellets and flake fish food.
Hope that help and if you are still worried feel free to PM me at any time!
callmoo