Most plecs are omnivores and will eat both animal and vegetable foods. A safe, general purpose diet that will keep most species healthy will comprise algae wafers, frozen seafood such as mussels, bloodworms, sliced vegetables (particularly courgette/zucchini), tinned peas, and bogwood (for fibre, essential for some species, helpful for all). Occasional offerings of things such as unshelled prawns and frozen whitebait are useful for the more carnivorous species (Acanthicus spp. for example) while true herbivores (such as Panaque) will need mostly plant foods in their diet to stay healthy.
No, they don't eat fish faeces. Neither do they "clean" tanks and anyone who sells them as "cleaners" is taking advantage of you. Mostly, they make tanks even more messy!