Scale. Cochineal scale, which favors opuntia is famous as a source of red dye, but there are other kinds of scale that don't produce anything red. Scraping it off as much as possible is good. http://ipm.ucanr.edu/PMG/PESTNOTES/pn7408.html

Thank you @lgteacher ! i wasnt sure if it was that because of the no red smear but i did not know there are some that dont produce it! thanks 😊

I‘ve had the cochinel on prickly pear cactus and they always bled red when rubbed off. It makes me wonder if that is really what you have. To get rid of them i sprayed with soapy water and this eventually killed them off. Good luck!