What’s this white residue in these leaves?
You have a colony of armoured scale, what you see is mostly adult females sucking on the sap of your plant.
Their shell protects them (to a degree) from many insecticides, but check with your local garden center what they have available. Regulations vary between countries. Apart from that, you should be able to wipe or gently scrape them off. Doing that dilligently and repeatedly may help as well.
That looks like damage caused by thrips, a tiny insect that has both nymph and adult forms. The nymphs resemble the adults but don't have wings. Here's a reference photo from Cornell University for comparison: https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/5755/files/2017/07/thrips1x2400-1x8drq9.jpg. It kind of looks like you have photographed some of the thrips' nymphs in the last photo - they'd be in the largest clump on the leaf.
Thrips are about 1/8" (1-2mm) long, light brown and cigar-shaped. They are difficult to eradicate because the adults tend to live in the terminal or leaf buds, so contact insecticides can't reach them. You might be able to get rid of the nymphs rubbing the leaves or washing them with a strong stream of water. Insecticidal soap may also help with the nymphs.
If you have other houseplants near the one with the leaf damage, move either them or this plant to a different location because thrips can fly (although they're relatively poor fliers).
Here's another link from Michigan State with another photo and some tips to get rid of them: https://www.canr.msu.edu/resources/thrips. There are many other resources on the web.