Mold spores are present in all soil and when the soil stays too wet, the spores will germinate and mold starts to grow. I’ve had this issue in the past, especially in the wintertime. Winter conditions in my home were the perfect storm for this situation until I took some action. I used to have my plants in the living room that faces northeast (too dark in winter) and where the humidifier is going 24-7 (too much moist air). Now, I move them to my back office in the winter (south facing = direct sun) and it is drier because I have the space heater going. The space heater helps with warm air movement and the soil dries out faster. I also started to bottom-water my plants in winter so that the top portion of the soil (where there are fewer roots to draw water away) doesn’t get wet. Lastly, I only water my plants when the soil surface is dry and there is no moisture whatsoever. You can also scrape the mold off of the soil surface and replace it with a ½ “ of clean sand which will not hold moisture. Do the scraping outside or in a garage because the spores will become airborne and can settle on unaffected plants. I’ve read that you can also lightly spray the soil surface with a 10% vinegar solution. Spray once in between waterings and wait a few days after spraying so that the vinegar has a chance to kill the mold.
I hope this helps if you have any issues with mold!