What is soil mold?
Soil mold is an encompassing term for various types of fungi that set up their home in the soil of your plants. This typically shows up as a white, fuzzy-looking layer on the surface of the soil and may smell a little like damp.
Mold is drawn to wet, humid conditions and feeds on decaying organic matter, so it's attracted to poorly-draining soils and / or recently-watered plants, but is very unlikely to affect dry soil.
Is soil mold harmful to your plants?
As soil mold only feeds on decaying matter, it's not interested in eating your living plants. Therefore, it'll stay in the soil and won't spread up to the rest of the plant, so you don't need to worry about it harming your plants.
If you do notice symptoms of mold on the leaves or stem of your plant then this is indicative of a different problem.
When is mold an issue?
Frequent extensive buildup of soil mold could be a sign that your plant is getting too much water and / or may not be draining well enough, so if you notice that it seems to be a persistent problem, you may want to adjust your watering schedule and repot your plant with fresh, well-draining soil. Increasing the airflow around your plant can also help prevent mold from re-establishing itself in the soil in the future.
If mold seems to spread from the soil to the leaves of your plant then this is a sign that it has a fungal infection of a more malicious kind, such as Fungal Rust or Powdery Mildew. Unlike Soil Mold, these fungi can potentially cause damage to your plant, so it's important that you treat them as soon as possible after noticing symptoms.