Well, it looks a lot like an armored scale insect if it is scale. It would really help if the genus or species of the host cactus is known, since many types of scale insects specialise in a particular plant. With the identity of the cactus, you could possibly narrow it down to the species of scale insect, but more importantly, you would be able to find out all the pests that that plant can have. There is an article here on Garden Web in the FAQs section that talks about how to best remove scale infestations. I hope this can be of some help to you.

At first it looked like a fungus because I couldn't see the bumps in the photo. You described scale. There are many varieties and they don't all look the same. Since you have it on a patio the normal pedators are not there to keep it under control. Horticultural oils or soaps will kill them. I have found that once you have scale it is very hard to "cure". When you have the scale under control you will need to monitor the plant because the scale has a habit of coming back. Also the scales don't drop off when they die so it can be difficult to tell if they are ALL dead.

hey welcome to GW .... first.. see link next.. is your product labeled for monthly use?? and that aside.. the mature ones are impervious to any treatment with their waxy coating.. and you must ID when the babes crawl out and are susceptible to the oil ... and that is once a year .. not monthly ... you are wasting product/money.. if nothing else ... third.. ants are usually around.. to 'milk' honeydew from whatever it is ... they are irrelevant to it all ... and the black is usually a mold as a result of the honeydew ... in other words.. get rid of the scale.. the ants will move on.. and sooner or later the mildew will go away ... the real problem.. is that the mature ones.. look no different if they are dead or alive.. just because you kill them.. doesnt mean they are going to disappear ... and frankly.. when it comes to this point.. i usually have a hard time 'looking' at them for years to come ... and it is then i think about cutting them down ... now i know nothing about this plant.. as i didnt even know it existed before your post ... but if it is like many hedges or flowering shrubs.. there is probably no reason you couldnt cut them to near ground level.. remove EVERYTHING.. treat the stumps for remnants of scale ... and within a few months.. just about have your hedge back .. since there is no insult to the root mass ... and in the meanwhile.. have removed 99.99% of the problem ... [i would burn it.. and find some satisfaction in sending them to the Elysian fields ...funeral pyre and all that ..]... hopefully you can find out if rejuvenation pruning' is an option with this plant .. good luck ken Here is a link that might be useful: link

Not scale - lenticels. These are pores that provide for gas exchange and pretty much everything in the Prunus genus has them. As well as many other plants.