It looks like some kind of skin infection. Sometimes these guys will scratch themselves up against a rock and irratation like this can happen.
Good luck with your goldie, and if this happens to another one I would definitely look into the pleco being a bully.
Hey sacrificetheory.. We don't know if the pleco was behind this. The OP will have to just consider that as a real possibility for himself. (Or not, whatever...) Either way, I'd like to stress one point here. It's much, much better for us not to characterize incidents like that between plecos and goldfish as a sign of aggression (the pleco being a bully). That couldn't be further from the truth and it casts plecos in a bad light that is totally undeserved. Plecos, by nature, are extremely peaceful fish and don't go after or attack unsuspecting goldfish (discus, etc..) out of aggression or being mean. They just like slime, whether it's a film of algae on the glass or that thick slime coat produced by many types of fish. Most of us who are familiar with and who appreciate plecos already know this, of course.. you probably do as well. But if we're not careful with our phrasing sometimes, newcomers just entering the hobby could get the wrong impression.