overfed fish
Overfed fish may be passing a lot of food undigested. In my cichlid tank, any falling poo gets attacked by blood parrots, there must be some nutrition left there I'd guess.
But, more likely the cory is finding dropped food and as he whisks the bottom is just breaking up the feces that you saw, now they are further down in the gravel, still there, rotting, you just don't notice them.
Step up the gravel vacs or get rid of the pleco.