Neon Tetra with fungus or some fuzzy material on back

[cemantic]

I went home for lunch and just noticed one of the Neon Tetras has this fuzzy stuff on it's back.  Gills and eyes seem unaffected.

There is:

10 Neons
12 Harlequins
10 Black Phantoms
2 other tetras
4 bushynose plecos

It is a 110 gallon fully thriving and planted tank.  Fully cycled no ammonia, nitrite, low nitrate.  Pressurized CO2 at about 30 ppm.  I add dry ferts regularly.

Tanks trimmed and clean once a week.  I do about a 25% water change at that time and dose with Prime.  This was done last Saturday.

All the fish have been in there for well over a month or two and everything has been fine until now.  I could only see it on one of them.

Suggestions?

I will try and get pics but it will be hard to do.