Something living on tropical fish?

[mini me]

Need some advice. Not seen anything like this before and web search not really helping. One of our glass catfish has something attached to its tail fin. Just appeared yesterday as far as I can tell. It’s not white spot as much larger. Looks like a small ball of cotton wool with kind of a strand/ tail trailing from it. No sign of anything on any other fish. Well established tank + 10yrs old. No issues for many years. Water tests all perfect. Any ideas what this is and how to treat?

Difficult to get image as he is constantly flipping his tail!

[Boosted LS1]

Probably has a small injury and that's fungal growth.