I have several types of crown of thorns, as well as other Euphorbia. I love how they pretty much bloom year round where I currently live and they are almost completely self-sufficient. There is one issue, however, that has been troubling me with their care. I keep finding masses of little black dots ringed in white on their leaves, both sides. I can rub them off and the leaf surface underneath seems to be largely untouched. I never find these dots on the stems and I have never seen them on any other plants, not even the leaves of my other Euphorbia.

Is it some sort of scale?