As others have said, it's some type of scale insect. A telltale sign is that they are mostly arranged on the middle line of the leaves, where, I imagine, it's easiest to hook into the plant's nutrients.

First thing to do is get it as far away as possible from any other plants you hold dear. Until it's sorted out, avoid cross contamination via your hands and tools as well. After that, you'll have to find some type of poison to kill them. Scales can be very resistant to external methods, something that is delivered via the plant's juices that they feed on might be easiest. Or you could spend weeks diligently rubbing the leaves and stems with alcohol to rid of them, to possibly no avail.

(I am not very fond of scales.)

They look like a sort of mealy bug to me. Spray isopropyl alcohol on them and if they turn red then definitely mealy bugs.

Get a cotton swab, spray it all down with alcohol and wipe them away and that should clear them up! Luckily they are pretty large and easy to see (at least more than other pests).

few drops of dish soap in water applied to the leaves will help this scale get evicted. you can spray it or apply it with a soft brush.

I found this reddit thread through Google lens, looks the same: https://www.reddit.com/r/plantclinic/comments/1hk3jab/confused_what_is_this/

tldr; some sort of scale bug

I highly recommend Cap Jack’s Insecticidal Super Soap

https://captainjacks.com/product/insecticidal-super-soap-rtu/

I'm sorry right now I can't hear what this YouTube video is saying but this looks exactly like your photo. I skimmed its captions and I don't think the YT video identifies it correctly. https://youtu.be/KEkk5gu4XKs?si=qq4It_utOm7UcrYa

Also, it looks like a mealy bug.