Ask Mr. Smarty Plants

From CactiCare.com here is an article on Pests and Diseases of Opuntia, including Opuntia ellisiana (Spineless prickly pear). We are not clear on what the film is on your cacti pads but scroll down the page and look first at Cochineal Insects and then at Thrips.

From a previous Mr. Smarty Plants answer:

The Plant Pathology Lab of Texas A&M has released a list of Opuntia diseases. Here are some extracts from that article:

Cactus Anthracnose (fungus - Colletotrichum (Gleosporium) spp.): This disease affects several kinds of cacti, Cereus, Echinocactus, Mammillaria, and particularly Opuntia (prickly pear). Infection results in a rather moist light brown rot which shows many light pink pustules on the surface. Spots are small at first, later enlarge and become covered by the small spore-producing pustules. Large areas may be affected, sometimes destroying entire plants. No satisfactory control is available, other than removing and destroying diseased cladodes as soon as noticed. In the greenhouse, soil from infected plants should be removed and benches disinfected. Spraying with a copper fungicide may help in checking the disease.

Scorch or Sunscald (fungus - Hendersonia opuntiae): This disease is common and serious on prickly pear cactus (Opuntia). Spots at first are distinctly zoned, later enlarging until entire cladodes turn a reddish-brown and finally die. The center of the disease area is grayish-brown and cracked. Other fungi may also be present in the diseased area. No practical control has been developed.

Scab (physiological): Particularly common on prickly pear cactus. Rusty colored, corky areas appear on the stems. Scab is thought to be a form of edema, resulting from overwatering and poor ventilation. Increase light and decrease humidity for control.

Stem Rot of Cacti (fungus - Drechslera cactivorum): Basal or top rot of seedling cacti that turns cactus into a shrunken mummy covered with brown spores. First symptoms are yellow spots. It can completely rot a plant in four days. The fungicide Captan should give some control.

Doesn't sound good, does it? We searched for pictures of the various diseases on a prickly pear, so you could compare them, but didn't find any. In fact, virtually every search phrase we used returned us to the article from Texas A&M that we have already referenced. On a couple of discussion websites, we found that Opuntia is considered by many an invasive weed, and no one seemed too interested in curing its diseases.

Summary: See each disease for suggestions for treatment, usually, cutting out the diseased portion and disposing of it in a way to prevent the disease being passed to others. Scab may be caused by watering the cactus, having it in too much shade and/or poor circulation. So, you could move it to a better location-now there's a challenge!