Answer: It isn't unusual for fungal growth to suddenly appear on houseplant soil. The moist soil surface (and water in the tray beneath) provide the perfect conditions for fungi to grow. This is especially true during winter months when fresh air doesn't enter a home. You've done the right thing in removing the soil and repotting your plant. Try not to overcrowd plants, or put them in a corner where air doesn't freely move around. To discourage fungal growth, empty water from the tray a few hours after watering your plant, and allow the surface of the soil to dry out between waterings.
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