The first time I saw this, I couldn't believe it. It's an infestation of some kind of wooly aphid that tends to colonise the Jade plants and it doesn't bother them that much but it's devastating to this cactus. What has happened is that the aphids have damaged the growth points and the cactus is doing its version of a cristate form. I don't really care too much, because it is a Hylocereus or night-blooming cactus, a listed invasive in my country and I need to get rid of it. This particular plant survives up on the rocks where it is extremely hot and dry. The fact that it has survived up there all along is proof of it's invasive abilities. I think the only reason why it is susceptible to this aphid is because it is so hot and dry up there and has become worse with our warmer weather and erratic rainfall patterns.
One of these days, I will get up and clear it all out and add it to the compost heap. My garden was such a neglected mess that almost all of the only plants that survived are listed invasive species. I made a list one day, I should look for it... But I counted over 15 species. I am doing my best to return to indigenous gardening, with plants that are suited to the extreme conditions here on the side of the hill.
It's especially important to control the spread of succulents because they are suited to these conditions and hacking out all the agaves is on my to-do list, once I get my backyard wasteland under control. The Vegetable garden is almost there after a back-breaking month of digging and sieving