WHY DOES THIS HAPPEN
If you notice a white, fuzzy coating covering the soil surface of your plant, it's most likely Soil Mold. It can sometimes look a bit like dust, but, unlike dust, it can also have a damp smell to it.
Mold likes humid, damp conditions, so if it shows up in your plant's soil then this is likely a sign that your plant has been getting a little too much water. It may additionally be a sign of poor airflow around your plant - this can prevent the soil from drying out properly in between waterings.
The majority of soil molds do not cause any harm to your plants, as they feed only on decaying organic material. This means they won't spread onto your plant - instead, the mold will stay in the soil. However, it can be unsightly, and it may indicate a more serious problem, such as overwatering.
Poor drainage can cause mold to develop on the soil. If you notice your plant's soil doesn't seem to be drying out properly in between waterings, it can be a sign that you need to change the soil, water less frequently, or invest in a better plant pot (such as one that has more drainage holes).
Try to also improve the airflow around your plant - this can help to eliminate any odors caused by the mold.