The lichen will not kill the willow but it indicates that the willow is not growing strongly. A strong plant provides shade and then the lichen is not as vigorous. Feed and water the plant to get it growing strongly.

Lichen is an indicator of clean, unpolluted air, and won't harm the Willow in any way. As Volunteer says, it simply means that the bark of your Willow is not growing or the Lichen would be unable to stay attached. It is very common on elderly trees whose rate of growth has slowed down considerably.