IDK what it is. You seem to be getting good help from Aquanero.
http://www.fish-disease.net/diseases/saprolegniasis.php
Saprolegniasis
Symptoms:
Visible symptoms are white or gray patches of filamentous threads that grow from an infected fish's body. It is characterized by an external, cotton-like appearance that radiates out in a circular, crescent-shaped or whorled pattern. Infestation generally begins on the head or fins and can spread over the entire surface of the body.
Cause:
The fungus Saprolegnia. Fungal spores may be transmitted by hatchery fish, wild fish, eggs, water supplies, and equipment. Saprolegnia is considered an opportunist facultative parasite, and so poor water quality, overfeeding, and low water temperatures may also play an important role in contraction of the disease. Saprolegnia causes tissue destruction and loss of epithelial integrity. If untreated, Saprolegnia will cause death by osmoregulatory failure.
Treatment:
Perform a 30% water change to improve water conditions and then treat the water with one of the following anti-fungal medications: Malachite green, Formalin, Hydrogen peroxide, or Sodium chloride.
Malachite green is considered the most effective chemical for controlling Saprolegnia. Formalin, a solution of 37% formaldehyde, is also effective and is the only fungicide registered for use in aquaculture in the United States; however, there are concerns about its effect on both the environment and those who handle it. Hydrogen peroxide is a promising chemical for the treatment of Saprolegnia with minimal impact to the environment. Sodium chloride can also be used to control Saprolegnia as it is lethal at high concentrations (15 gm/liter).
If the patches on your infected fish appear gray, treat with a broad spectrum antibiotic, as this is indicative of a bacterial presence.
TBTB: Hence, Tom suggested Triple Sulfa.