Lymphocystis?

[Hybridfish7]

Have you tested your water?

Yes

If I did not test my water...

1. ...I recognize that I will likely be asked to do a test, and that water tests are critical for solving freshwater health problems.

Do you do water changes?

Yes

If I do not change my water...

1. ...I recognize that I will likely be recommended to do a water change, and water changes are critical for preventing future freshwater health problems.

Just want to confirm. I know it's not the best picture but I can't think of anything else that grows like that.

 

[Deadeye]

Based on the look, I’d agree with lympho.
Just to throw out another option though, it could be damage/infection from fighting with another fish. Keep an eye on it to see if it gets better or worse.