When a cactus molds, it typically indicates an issue with excessive moisture and poor air circulation. Mold growth on cacti is commonly caused by overwatering or high humidity levels. Mildew appears on the cactus's surface as fuzzy or powdery patches of various colors, such as white, gray, or black. It can be harmful to the plant if left untreated, as it can lead to rot and other fungal infections. To address mold issues, it is essential to reduce moisture levels by adjusting watering practices, improving ventilation, and ensuring proper soil drainage. Additionally, affected areas can be treated with a suitable fungicide to combat mold growth.