I recognize that stuff. Bought a NTT from a overstocked tank and it came with something similar. I have seen it in other peoples tanks as well. It seems to occur when the water parameters drop off and mainly targets perch-like fish.
The way I cured my sick NTT was to keep my water perfect, rise the temp to 30c and add 2dl salt per 100L tank water.
It heals slowly as all hell but it's curable.
Best of luck