It looks like spider mites to me. I have a similar issue and I'm using neem oil to spray all surfaces of the plant every 1-2 weeks until they go away. There are some other treatments you can look up too.
Agree with spider mites, pretty sure I can see webbing between the leaf tips.
Neem will work, so will insecticidal / horticultural soap and it smells less bad. Heck they’re soft-bodied, you could probably spray 50% isopropyl and kill them too. I like the soap because it’s really cheap and safe for pretty much anything without farina / epicuticular wax (I feel like some begonia Rex aren’t a fan either).
This is a good sized infestation so I would recommend starting by rinsing the leaves really well in the shower or with a hose. After debulking the colony, then use insecticide.
Check your other plants too OP! Mites are not particularly particular.
Definitely spider mites, I've gotten rid of them by giving them a good spray down with my shower head, wiping all the leaves with damp paper towel, and then dousing the plant in miticide (i used safer's end all, it comes in a yellow bottle). I did 3 separate treatments 3 days apart. and then kept them quarantined for a month to make sure i got em all. I managed to get rid of the mites on most of my infested plants except for my parlor palm which i ended up throwing out. There's other, more environmentally friendly options you can google but that's what worked for me.
Make sure you get all the nooks and crannies, including the underside of the leaves, stems, and the top of the soil. Those buggers are prolific and if you miss some then they come back quickly.
In addition to what others have said (neem oil, keep it quarentined) also make sure to clean and sanitize the surrounding area and any area its been in recently to avoid reinfection.
Also mites hate airflow. Keeping plants in spaces that arent too closed off can help discourage mites from setting up shop.