HLLE wouldn't show up overnight like that. It isn't uncommon for these fish to suddenly have odd color changes in a given spot. My old one would occasionally have a white spot the size of a dime on on side of its body--it would literally disappear anywhere from a few hours later to the next day. In your case, it could have scuffed up the scales in the vicinity (highly probable given this species' behavior). From the picture, it doesn't look like any sort of infection, either--so for now, I would just observe it and stay calm. I don't think it will be any real issue.
BTW, this species really isn't that dependent upon algae and greens. Giving them too much may not help, actually. They are largely planktivores in the wild and would benefit from good, meaty, proteinaceous fare, like mysids in the aquarium. However, algae and green matter should still be provided--it just shouldn't take precedence as it does in other species.