The vast majority of mold that appears in plant soil is harmless to your houseplants, but it can still be an annoying unwelcome visitor!
SIGNS YOUR PLANT HAS SOIL MOLD
A white, fuzzy or dusty-looking covering appears on the surface of the soil
The soil may have an odor to it
Most molds that appear in plant soil are not harmful to your plants - they are a type of saprophytic fungus, meaning that they feed on dead organic material. Therefore, they won’t feed on your plants and so won’t cause any direct damage to them. In fact, sometimes they can be beneficial!
However, soil mold can be unsightly, and can sometimes be indicative of a larger problem, such as poor drainage or overwatering.
WHAT TO DO NOW
Usually, it’s enough to just scrape the mold from the soil surface, then allow the soil to dry out - try to also place your plant somewhere with good airflow if possible:
Manually remove all visible mold
Cut back on watering for a while - let the soil dry up between waterings and water a bit less each time
Always check the soil before watering - in the Info tab on your plant’s profile, you can find how dry the soil should be before giving more water
Sprinkle some cinnamon (which is antiseptic) over the top soil. You can also buy recommended treatments for mold
As well as causing mold to form, excessively wet soil can lead to greater problems such as root rot, which can be very difficult for your plant to recover from.
HOW CAN I PREVENT THIS FROM HAPPENING AGAIN? As molds develop in damp conditions, try to make sure you’re not overwatering your plant. If the plant type allows it, then you can try letting the soil dry out between waterings (check in the plant’s Info tab to see if this is okay for each particular plant).
HOW CAN I MAKE THE SOIL ODOR GO AWAY? If your soil has an odor, exposing it to better airflow and allowing it to dry out will help it to go away faster. Once the soil has dried out, the odor should be more or less gone, or at least much less strong.