How many gallons is the tank please?
How many fish and which type?
Can you post your water stats in ammonia, nitrite, nitrate, and ph.
White fungus on the mouth is Columnaris.
Any signs of the mouth rotting away?
You will need to treat with a bacterial medication. What is your location please. To help you with which medication to buy.
When using medications increase aeration. I would also advise lowering the temp so the bacteria does not multiply so fast.
some info.
COLUMNARIS
The hospital tank should be heated to approx. 74 degrees. 76 and above is the ideal breeding temperature for columnaris. Though there is some dispute over lowering the temperature, my experience has been that 72 is too low for the medication to work rapidly, 76+ causes the disease to breed more rapidly than the anti-biotic can kill, and 74 is "just right." Remember to keep this temperature stable!
Text © Lauren Weeks
Maracyn a good product for Columnaris. But it will wipe the beneifical bacteria out in your filter.
Maracyn, Maracyn 2, Maracyn Plus.
http://nippyfish.net/sick-betta/cotton-wool-disease-flex/