Try to take your pic with a high flash setting, and shoot it on an angle with as much zoom as the camera can handle, while still producing a good pic. This way you won't get the direct white beam, and it'll be easier for people to ID the problem
That looks to be fin rot. If your water conditions are good you can treat with melafix. I had the same thing happen to my clown trigger and it cleared up after 3 days. Good luck.
