i doubt that the cory cats are the source of all that poop--it's probably just your bristlenose (ancistrus). all plecos are famed for their pooping ability and the crud shows up especially well on sand
reducing feeding won't really help too much since your bristlenose is going to eat as much as he wants anyways and only the cory cats will be deprived.
you've got a couple of options though:
--clean constantly (probably the least fun idea)
--add ground cover plants to hide the poop (i'm a big fan of pygmy chain sword)
--add some tunneling Madagascar Trumpet Snails (they eat nutrient-rich big poops and turn them into concentrated little poops. they also like to dig around in the substrate and help keep it aerated. the best part about MTS is that they don't eat plants!)
also, try feeding in locations that are inconspicuous. my plecos have always pooped the most in the places where they feed and the places where they hide. you may not be able to reduce the amount of pooping, but you can encourage the pleco to not poop right in the center of the view!