i cant think that the moss as i see it is hurting your willow i think at all . it is all over my verigated acer tree and its grown in 11 years about 18-20 foot ish . its not a moss either its a likek i believe and yes it uses the tree without hurting it thow . a better picture mite be helpfull however as its not tyhat clear realy .

Can't really tell what's on the trunk, not close or clear enough but its probably lichen, doesn't do any harm, is often present on older trees. If you don't like it, wash it off with horticultural soap and water.

No, don't wash it off, it is a natural and interesting feature of the tree. Sorry Bamboo. It shows that you have good, clean air.

I agree Bulbaholic, and didn't say it should be washed off, only that it could be...

we are all in agreement i see lol x .