They're all one kind of critter. But different developmental stages.

They're aphids, not mealybugs. And yes, some have a waxy covering, even more so than those on your leaves.

What plant are they on?

It's a small elm tree and there are mealybugs on the tree as well. I watched adult mealybugs almost 1/4" long crawl out of that mess when I tapped it with a stick.

Wooly elm aphids in various stages of life. That's all I see. It could be easy to mistake a mature wooly elm aphid for a mealybug.

There are lots of species of wooly aphids, by the way. This seems to be the time of year when we see a lot of them.

Upon further inspection you may be right about the pictures above but this is not a wooly elm aphid. 4-5 of these were also found on the tree.

I also found many leaf galls which could be from beneficials.

After a rather tedious count, I came up with 3,427 give or take a hundred! Last picture looks like some type of elm pocket gall. If so, from a type of mite that causes little damage to the tree.
hortster

The critter on the blue background is a beneficial insect. Likely a syrphid fly larvae; They kill aphids. The aults are also called flower flies or hover flies and, because of their markings, resemble bees.

You said you " watched adult mealybugs almost 1/4" long crawl out of that mess"
If that critter had a shaggy white waxy covering, they were most likely a coccinelid larva- in the same family as ladybugs you see every day. One kind we have around here is Cryptolaemus.

The leaf galls are one of those things that nature does. Seldom if ever damaging to the tree's health.

No jean it was not a mealybug destroyer/wooly lady beetle. If you'll remember I posted a picture of such last year.

Here is a link that might be useful: Mealybug or predator