If it's indeed fungus (and not just algae), you could try any of the below methods:
From the Aquatic Plant Q&A at the Krib:
"How do I disinfect my plants?''
New plants may have unwanted hitchers: snails, algae or disease. Disinfection can help reduce their transmission into the tank, and can be used to remove algae growths from established plants. Beware, there is always a danger of going too far and damaging the plant itself. Some popular methods:
- A ten minute soak in potassium permangenate (pale purple) works well; it is available in dilute form from Jungle products as "Clear Water". Permangenate is particularly good for killing bacteria and pathogens.
- A 2-day soak in 1 tbsp/gallon of alum (buy it at drug stores) is good for killing snails and their eggs.
- If the plants are kept in a fish-free system for three weeks, parasites like ich and velvet will die without their fish hosts.
- A soak in a 1:19 diluted bleach solution; 2 minutes for stem plants, 3 minutes for tougher plants. Make sure to remove all traces of bleach afterwards by rinsing with water and dechlorinator. This method can kill your plants, so use only as a last resort against hell...o algae.
