Mold occurs in houseplants because of too much heat and humidity in the house. Meaning you have either watered the plant too much or it is in a poorly ventilated area or both. Plants in the washroom/toilet are especially prone to this. All those hot steamy showers will do it.
Having moldy plants is also deadly for your health, especially if you have a central heating system that circulates the air in the house. This is a disaster that will get you sick. We are talking of sickly strains of disease that will affect your lungs that could have serious long term consequences. Mold spores can cause allergic reactions, especially for asthmatics,
If you love indoor plants, here is the best way to deal with it.