I've seen cuts and bruises, banged-up and missing scales, but something about this in particular makes me a bit uneasy. It looks like some kind of abscess, possibly infected, and is reminiscent enough of uronema to make me want to get some advise and double-check.
It's almost impossible to see when looking from any angle that's not overhead so it's possible, but kind of unlikely, that it has been there more than 24 hours, but almost impossible that I would have missed it for more than a day.
Anyone have any thoughts or advice?