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.