They look like scale or another related insect that comes in fizzy/waxy species. Doesn't matter too much what exactly they are--for houseplants it's probably best to just scrape them off whenever you see them and hope you get enough to stop the lifecycle. If the plants can take it, putting them in the sink and washing while wiping is even better.

Of course if they are really big or delicate plants it might not be as practical. I suspect any insecticide or insecticidal soap you can find for scale (or adelgid or aphids or other sucking insects) would work.

In your first photo, those look like mealy bugs. i had them in a house plant of mine as well.. the tinier white spots are eggs. take q tips dipped in alcohol and go leaf by leaf and gently wipe off. check your plant every 3-4 days for a month or 2. they reproduce like crazy and pay special attention to crevices where they like to hide

Im not sure about browning on stems but white puffs all look like mealy bug to me. just be persistent, youve got this