Help in identification of pest

[Charon]

Hi, this is my first post here, I hope you can help me.
Few of my trees are infested with something I cannot identify. They are wooly and come off easily, however it returns after few days. One tree excreted something brown after I scraped it off. It seems to somewhat retard the tree growth. My guess is they are some aphid or some kind of fungi, however I cannot find what it is exactly and how to treat it effectively.

This is the walnut tree

imgur: the simple image sharer

and the lilac

imgur: the simple image sharer

imgur: the simple image sharer

[veggiechicken]

Hello Charon and welcome.
Could you add your location to your profile please? This would help us to narrow down the types of infection that your trees may have.

[Martin H]

They look like woolly aphids: Eriosomatinae - Wikipedia, the free encyclopedia

Quite a spectacular colony!

Some thoughts on control here: https://www.rhs.org.uk/advice/profile?pid=724

[Charon]

Thak you
I've scraped them off with cloth, just hope the little buggers don't return