UltimateBettas has a very informative common disease section. Here's a link to the thread:
Fish Disease Treatments - UltimateBettas
The part you may be interested in is this, I don't know exactly which of the two of these you may have, but here's the medications for some symptoms. Decide which sounds the most like your situation, then treat accordingly.
Symptoms:Thin, translucent/fuzzy white patchy growths = Columnaris disease. This will eventually start to peel the skin off in strips, leaving a bloody red area on the fish.
Use: Any sulfa drug combo, like Triple Sulfa, Super sulfa, or try some aquarisol or copper safe.
OR
Symptoms: Stringy white cotton like growths on fins or body = Saprolegnia Fungus. If neglected the cottony area's will disappear and turn into severe red sores which eat away the fins and/or the skin.
Use: Forma Green (from Nationalfishpharm.com), Nox Ich or Quick Cure.
Hope this helps!
Brinny