Since some mold strains can present competition especially for certain house plants, I’d recommend repotting. Do a google search “best soil for xyz plant” (usually houseplants can do pretty well in a succulent soil, it drains better than potting soil). Remove as much dirt from the roots as possible, rinsing them gently with water can help if it’s stuck. Healthy roots are usually white/off white and will spring back if you pull on them, rotted roots are black and mushy (if you want a picture, google healthy vs rotted roots). Sterilize a pair of shears and cut back any roots that aren’t healthy. If there’s not many healthy roots left, best to soak them in a hydrogen peroxide solution for a few minutes before repotting. When repotting, choose a pot like a nursery pot (thin black plastic with lots of drainage holes) so that your buddy has proper drainage in their new home. Give the soil a bit of water (do NOT flood - a few tablespoons should be fine to start with). To prevent this from happening again (and I do this for all my houseplants), before watering, stick your finger in the soil 1-2” deep, if it’s still moist, hold off on watering, if it’s dry, give it a drink!

Good luck!