If the plant was outside and then brought in then the spores probably came in with it. Even if not, there are mold spores in the air even in the house that can float in the air and land on the soil. Most plants shouldn’t Be kept so damp that mold can grow so I suspect that you are overwatering. Plants need far less water than most folks believe. If there is a saucer under the pot, it is there only to protect the floor or woodwork…not for any benefit of the plant. NEVER let water stand in the saucer. When you water, any water in the saucer that doesn’t get drawn up into the soil within 5 minutes or so should be dumped. Now that the mold is there stop watering and let the soil dry. Take a pencil and scratch up the surface of the soil to help it dry out. Keep the surface of the soil dry by light watering and pour the water into the saucer below and let it get drawn up from below. And discard any that isn’t. Less is more when it comes to houseplant care. These molds aren’t usually a problem for the plants but the drowning conditions are the real culprit.