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.