tumor (?) on corys

[Phoenixphire55]

So, I have four green corys and a few weeks ago I noticed that one of them had a white blob thing attached to the back of his dorsal fin. It looked like a little white ball and I assumed it was a tumor or something, and after a week or so it started to go away. Now today I just noticed that three of the four have the same exact thing in the exact same spot. Pretty weird. Any ideas?

 

[Zagz]

I've seen things like this come and go on fish. Unless it gets bigger or causes problems I just watch it and don't worry too much.