looks like you got a cory cat, or did you aleady have that?. . .
if you are truly set on getting an effective algae eater, then an oto (otoclinus cat) would be your best choice, they stay small and are quite good at nibbling algae, though they do need supplemental food.
aside from being overstocked, which you already know, the problem with having one cory is that they get shy and listless without others. they need to be in a group of at least three (which you shouldn't attempt at this point) of the same kind to be happy and in the best health. maybe it's time to trade in the cory for an oto, or just some future store credit? i'm not trying to attack you, but you know there's a problem, well, a couple problems, and adding more fish will just make it worse.