I presumed these were aphids covering a branch of a native beech alongside a mountain trail, but I’ve never seen anything like it. Quick research tells that they are beech blight aphids, known commonly as the boogie woogie aphid. They are typically still while feeding, but they sway frantically when approached. I am easily entertained, but this lightened the load of the uphill hike.
I am very forgiving of insects in my garden, but I’m not certain I’d be able to overlook this invasion, even if the aphids will do no more than kill a single branch of the mature tree.
Ramblin' through Dave's Garden 
Who knew! Thanks for sharing that video- entertaining in a creepy way! Glad we don’t have any beech trees on our property.
I have several beech, but I guess fortunately I’ve never seen this aphid.
Wow, that’s so funny! I’ve never heard of these aphids, or seen anything like that. (Probably I would not find it so amusing were they on my own tree!)
If the aphids were on one of the beech in the garden I would have to check back regularly to see if another insect appeared to keep the aphids under control. A different aphid dependably appears as milkweed flowers decline, but then a small black beetle appears that eats many of them. Some years the milkweeds collapse from the aphids feeding. This year they didn’t, but they always come up the following spring. The natural processes fascinate me.
Oh! That is disturbing! That would keep me away.