I have to disagree with you on that cories do not eat algae Azurescriber since cories, one of their diet consists of algae. However, they are bottom feeders and will only remove the algae and uneaten fish food on the substrate. If you would like to see some prove then here's a website link:
It is on #8 of the 10 ways. Also, it also tells about how to control the algae if that helps.