There are a number of different fungal diseases that can affect your plant, which vary in severity.
HOW DO I KNOW IF FUNGUS IS HARMING MY PLANT?
Depending on the type of fungus infecting your plant, it may show symptoms such as damage to the leaves, marks on leaves, premature leaf drop or yellowing of leaves. Therefore, if your plant seems to be showing signs of damage, it could be that it is suffering from a fungal disease. These usually occur if conditions have been especially damp for prolonged periods of time, so take care not to overwater your plants and try to make sure it has sufficient drainage.
However, it’s also possible that the fungus is totally harmless. For instance, if you can only see evidence of mold in the plant’s soil (and you don’t see any mold on the actual plant), it is unlikely that this will damage your plant, as this type of fungus feeds on dead material found in the soil, i.e. it doesn’t eat plants! Although, it may be worth noting that there is a potential risk that it can cause some stress to your plants, as it can leach away important nutrients if left untreated, but is very rarely ever fatal.
You can also try spacing your plants further apart from each other, to encourage good airflow and help prevent fungal spores from spreading from plant-to-plant.
WHAT TO DO NOW
Isolate the infected plant to prevent the fungus from spreading.
Make sure that the pot your plant is in has good drainage and that it has holes in the bottom.
Try to remove all infected material and destroy it completely to help stop the spread.
Consider using a fungicide. There are several good ones on the market, and some are even organic. Make sure to always follow each brand’s specific instructions.
You can also use Neem oil.
Check all of your plants from time to time to make sure it hasn't spread and to generally keep an eye out for any symptoms of poor plant health.