White powdery tree fungus on bark?

Not insect like mealy bug? More like bits of cotton. There are a variety of tree fungus that cause white powder on bark, we'd need the tree species first and the overall health of the tree. Is this on the bark or did the bark slough off and the fungus underneath? None is very good. You need to have a proper diagnosis from someone in your area who knows what diseases are common, knows tree species and they can help with decision to treat or remove tree........some fungus shows up in late stages of tree decline.

Make sure you're not seeing lichen:

https://www.google.com/search?q=diva+hydrangea&hl=...

If it looks like that, it's harmless, it taking nothing from the tree, and is merely hitching a ride.