Looks like a small form of scale. It is a critter that steals juices from your cactus and damages the epidermis while hiding under a scale. Perhaps a q-tip with rubbing alcohol would be good for dislodging the insects and killing them. Personally the few times I've had this on one of my cacti I didn't get it under control quick enough so I might not be the best for giving advice on how to treat it.

Yep I agree scale🙁. You can also use a soft toothbrush and dip it in rubbing alcohol. You will need to do this treatment about once a week for 3-4 wks because you will always miss one or two. Really focus on the areoles because that their secret hiding spot. Once you notice they are gone for good change the mix and scrub the pot with soap.

Put your cactus in an area away from all your other plants, and check your other plants while you’re at it.

Thank you so much @polyp and @Kara! I feel so bad for not figuring out the problem sooner... will definitely carry out the treatment and hope it’s not too late for me to save the cactus. Fingers crossed. Thanks again!