If the pleco is the Common Pleco (Hypostomus plecostomus) then plant matter only makes up a small part of their diet. They're the most common plecos sold as "algae eaters" but they're true omnivores, and will eat protein rich foods. They're probably one of the worst "algae eaters" out there. But for food I'd recommend a general tropical sinking pellet.

Just keep in mind this fish can well exceed 18"+ and can be quite messy.

And just a note, for the true algae eaters, the nuisance algae grown from too much sunlight is very poor in nutritional value, and should be supplemented with an algae wafer, leafy greens, or slices of fresh veggies.

If you need an algae eater, I'd personally recommend the Bristlenose pleco.