I don't think it's a parasite, in my case, Whiskey doesn't flash or dart or do anything I've read about with infestation. He acts perfectly normal. He eats, he swims, he poops normally, he sleeps. He seems utterly fine.
It doesn't have to be a tumor though. If it had scratches, maybe he injured himself and he's got an abscess under his skin? I'd keep an eye on it and if it gets red or raw looking then address that issue. In the meantime keep things clean.
Oldfishlady may have more to offer on the issue of wounds. I only really have experience with Whiskey's particular issue. I've never had a wounded fish and don't really know what to do or what to look for.