Indoor plant soil offers a near-perfect environment for fungal growth, particularly if the soil is rarely allowed to dry out. In most cases, fungus that exists only on the soil will not harm the plants. However, affected soil can release spores into the air, which may trigger allergies in susceptible people. For better air quality in your home, and the general appearance and well-being of the plants, you should keep your houseplants free of.
Fungus on indoor soil is often black, yellow, white or gray and rarely affects soil more than an inch below the surface. Fungus may appear as fuzzy mold over the entire soil surface or in patches. Yellow mushrooms may also appear, if soil is infested with a particular fungus called Leucocoprinus birnbaumii, according to Tom Volk of the University of Wisconsin.
When soil is kept continuously damp, fungus spores will very likely grow on the soil. The soil provides a rich food source for fungi and, as long as it is kept moist, the fungus will thrive. Decaying matter, such as dead leaves or petals left on the soil, will also contribute to fungal growth on the soil.
Fungal growth on indoor soil should not harm plants, but keeping plants too moist may contribute to plant diseases such as root rot and may invite fungal diseases to plant foliage. It may cause an unpleasant musty odor around the plant, however, and looks unattractive and unsanitary.
Remove the affected layer of soil, adding fresh soil. You won't need to repot unless the fungus keeps returning--in this case, the entire pot may be infected and you'll need to discard the old soil and decontaminate the pot with a bleach and water solution (the standard 1 part bleach to 9 parts water). Use fresh, sterile potting soil when re-potting.
Place the plant in a sunnier location of the home, if possible, and allow the plant soil to dry out in between watering. Most houseplants can tolerate this. Keep the plant's soil free of dead leaves and other debris so fungus has less of an organic-decay buffet to feed on. Good indoor airflow via fans may also help prevent fungus by keeping the air around plants drier.