I wouldn't worry about it. In fact, I think they are supposed to have it. If it was a disease, it would be all over the body, not just in one line. I looked on the internet and you have a point there, they normally don't have that thin line. Maybe it just got a unique beauty mark or else it is just a mutation in the genetics. Like I said, I would not worry. Unless it starts dancing out of the tank, I don't think you have a problem. It is pretty by the way!
