Why does my cactus have white stuff on it and how can I get rid of it?

The white stuff on your cactus is likely powdery mildew, a fungal disease. To get rid of it, you can try wiping the affected areas with a cloth soaked in a mixture of water and mild soap, or using a fungicide specifically designed for powdery mildew. Make sure to improve air circulation around the plant and avoid overwatering to prevent the fungus from spreading.