That looks like Lymphocystis to me....
If so, not necessarily fatal, is viral, can be treated as an infection.
Hopefully we can get a few others to chime in here
Check sticky notes for reccomend meds....copper I use for parasites, so I have no idea if that will work. I was thinking more like a general prophylactic.......in any event don't mix them.....
In addition, feed high protein foods, frozen meaty stuff.
Make sure your temp is between 77-79, too warm water has been discussed as not a cause but a contributor, same with water quality and nutrition......
That's assuming the diagnos is correct. Just my two cents