Willow Tree Growth
Willows often grow advantitious roots, or air roots, which is what these are. This is typical of many plants that grow in wet environments and is the natural way that they seek to develop secondary roots to stabilize themselves. You do not need to do anything. Usually they will grow and dry out in a few weeks or you can pinch them off with your fingers without harming the tree. Do not use a sharp implement that might injure the bark.