I feed mine Tabimin tablets as their main food, supplemented with bloodworm, daphnia, peas, spinach, broccoli. Veggies are good but shouldn't be their main food. Mine have never even looked at the algae.
Basically, you should count on feeding these guys. They will eat leftover food, on the other hand, there shouldn't be lots of leftover food in a cory tank as their barbels are really sensitive to a dirty substrate and they can get bacterial infections (usually fatal). You need to be VERY careful with this if they are in a cycling tank, do frequent water changes and gravel vacs.