Plecos mainly eat algae and they will eat all the algae that is in the tank they are in, but that does not mean that you should not feed them other food.
Any aquarium will not have enough algae present to feed the plecos all the time, since you need to filter the water and change it. Your plecos will starve if you think this way.
So, what else can be pleco food? You need to keep in mind that these fish are omnivores, so they can eat both vegetable and meat-based foods. Some of the other foods that they can eat are driftwood, bloodworms, earthworms, certain types of vegetables, shrimp, and a variety of other things.
If you are not 100% sure what to feed your plecos, it is advisable that you do your homework before just giving them anything.