I get that on caladiums and St Vincent's Lilac....I thought it was white flies.

Whiteflies can congregate like in the photo but you should still be able to view them as distinct insects rather than a furry mass. Whiteflies also tend tend to scatter or fly when disturbed. Mealies just glom on and never seem to move :-)

But they are closely related and the damage is very similar, as is the treatment for control.

So, the winner was mealybugs. I saw them today and they look exactly like the photos. They were on the bottoms of the leaves, right along the main stem. They suck the juices out of the plants. I shot them all in the face with insect spray...let them suck THAT! I hope my plant survives.

There are several varieties of mealybugs too. They don't all look the same. Don't whitefly larvae do this as well?

I usually just wash them off with a strong spray of water if there are a lot. If it's a small area, I wipe them off, smushing them.