Corys aren't algae-eaters, you'll need to feed them. Mine eat several types of bottom-feeder wafers, as well as frozen bloodworms, flakes, and anything else they can get into their teeny mouths.
Plecs should also be spot fed IMO and not left to survive solely on algae. Most are omniverous, some are quite carniverous in nature.
Algae wafers, veggies like zuchinni (courgette), potatoes, beans and peas, along with bottom feeder wafers and even occasional meaty meals will keep your plec happy and healthy.