This is scab. Usually from overwatering and poor ventilation. Increase light and decrease humidity. Overwatering is a common cause of houseplant death. Good luck.

There are several diseases that attack prickly pear. The usual cause is a too moist environment, and allowing the plant and soil to dry completely between watering may help. However, some diseases are fatal and can spread to nearby plants. Here is INFO on the most common.

Start by separating this plant from others. Keep the pads dry; a south-facing window will ensure enough sun while avoiding rainy weather. Because it can be difficult to tell the difference between pathogens, I suggest you spray the plant with Physan 20 available HERE as well as from Amazon. Physan 20 is powerful and you should wear gloves, goggles, and a long sleeved shirt while applying, but it does work against serious plant diseases. Repeat application after 10 days.