A white or yellowish substance on the surface of the soil of a potted plant is saprophytic soil fungus, a mold that feeds on decaying matter in the soil. While it is most often noticed on houseplant soil, it may also grow inside bags of potting soil you have stored in a damp area or an unsealed bag. The mold rarely poses a danger to the plants, but it is unattractive and its presence indicates other issues with the soil that may harm the plant or plants grown in the soil if not treated.
Check the soil moisture of the plant. Slide a pencil into the soil, then pull it out. If clumps of soil stick to the pencil, it is too moist. Allow the soil surface to dry slightly between waterings.
Sterilize the potting soil before use if mold is present in the bag. Place a 4- to 6-inch layer in a roasting pan and heat to a temperature of 180 degrees Fahrenheit for 30 minutes.
Remove the plant from the pot if the mold is severe or doesn't go away within a few weeks after adjusting moisture. Pull the plant, soil and all, from the pot and set aside.
Rinse the pot in a solution of 1-part bleach to 10-parts water. Rinse with clear water and pat dry.
Shake as much soil from the roots as possible and make sure to remove any visibly moldy areas. Loosen the roots on the bottom of the plant with your fingertips. Dispose of this soil or sterilize before reusing.
Place sterilized soil in the pot and replant the plant to the same depth it was at previously. Continue to allow the soil to dry slightly between waterings so the mold doesn't come back.