Mold on your houseplant's soil won't usually harm your plant directly, but it does indicate a less-than-healthy growing environment. Here's how to fix it and provide better living conditions for your plant.
The mold that spreads over topsoil are types of fungi and they come in many different forms. They can vary in color from black and white to green, brown, and orange. What they all have in common is the ability to reproduce by releasing spores. Because this allows them to spread very quickly, including to nearby plants, it's important to take action as soon as you've spotted your unwelcome guest. Immediately isolate the affected plant so that the mold does not spread.
To get rid of the mold, remove the top two inches of soil, wash your hands, then replace it with fresh material. Water in the soil. Next, scatter the top of the soil with ground cinnamon, which acts as a natural fungicide. It's best to use fresh soil from a newly-opened bag. This way, you can be certain that this new potting mix is not uncontaminated with the same fungi.
A more drastic measure would be to remove all the soil inside the infected pot, sterilize the pot by washing in hot water, and fill it up with brand new soil. However, some plants do not like being disturbed too much so if you feel a complete soil overhaul is necessary, it's best to do this during the growth season when plants can more easily recover from the shock.
There are several things you can do to keep fungi at bay.
First, you can use a soilless potting medium for your plant. If soils contain a certain ratio of decayed organic matter to mineral particles, soilless mediums are mineral through and through. The most popular soilless potting mediums are vermiculite and perlite, both made from volcanic rocks. Mold will find it almost impossible to proliferate on these potting mediums because they do not retain excess moisture like soil does.
Succulents, cacti, and some tropical plants will really appreciate a soilless medium because they offer light, dry, and airy conditions. However, other tropical plants like monstera, philodendron, and ficus do need the nutrient and moisture-retaining properties that can only be provided by soil. In this case, you can just mix perlite or vermiculite into the soil to reduce its moisture-retention or you can encourage better air circulation in your room.
Too much air humidity can lead to mold outbreaks, although this usually only happens when accompanied by a lack of sunlight and air circulation.
Putting your plants in a sunny position near an open window will prevent mold outbreaks. Sunlight is the most prevention for mold while the natural breeze will encourage evaporation and regulate air moisture. If you have a plant that does like a lot of air humidity, you can put it in an airy spot while raising moisture levels immediately around the plant by placing it on a tray of wet pebbles.
Check your watering
Another factor that encourages mold is overwatering. Cut back on your watering regime if your plant is showing limp leaves. Using clay pots rather than plastic ones might help in this. Clay contains tiny pores that will suck any excess moisture away from the soil and cut the chances of root rot. Apart from this, clay pots are more sustainable and look better!
If mold keeps returning, you can scatter a little natural fungicide onto the topsoil. Neem oil, apple cider vinegar, cinnamon powder, and baking soda are both less toxic and often equally as effective at mold-removal as synthetic formulas.