For what it's worth, "mouth-rot" isn't a fungus. It's bacterial. In my experience, it attacks live-bearers and catfish the most. It used to knock off my pictus cats when I was a kid :cry: (stress, generated by high nitrites - before I learned about proper bio-filtration).
I've spent some time tonight and studied that small growth on my rasboras. I really have no idea what it is. It isn't 'fluffy/hairy' like mouth-rot. As far as I can tell, it is an extension of the fishes 'skin'. Not a foreign growth.
I ran about every search possible through google, but couldn't find any info. If you are ever offered an explanation, please share. Until then, I'll just leave em alone and keep a keen eye on em.