I've never heard of internal fungus', it's probably either overbred
white mollies (I seem to remember having read that mollies are
naturally black) as stated by sophie, or a bacterial infection, ask
around and maybe try a full dose (so the bacteria doesn't get the
chance to evolve and adapt to the anti-bacteria medicine and become
resistant) of an antibacterial medicine. Remember to research the
medicine before you use it as you might very well ruin/damage a
significant number of lifeforems (including good bacteria) in the
medicated tank and try to use a hospital tank after researching the
pros and cons of such too. Good luck!