If it's white cottony stuff on the pads, there's a bug called cochineal scale inside that you can crush and it will release an intense red color. If the bugs are more hard and firmly stuck on, it's a different kind of scale. Treating the first is easy: remove all the bugs from the plant (eg. by hosing it down carefully) and repeat in a couple of weeks. Treating the second is a bit more effort, usually some brushing with soapy water will get them off. In either case, once you get rid of them the plant will get better, and you need to pay attention and repeat the treatment if they come back. Also consider treating the whole plant with a systemic (insecticide that you water into the soil and is taken up by the plant, conferring resistance). This works much better in a preventative sense than as a treatment (physical removal is far better as a first step).
