Willow tree fungus
I can't identify the specific fungus this is but is very likely saprophytic and only feeds on dead material. Over time this will weaken the tree and make it more likely to topple during wind or ice storms. If it is in an area where it is likely to damage nearby structures or be a danger to people if it fell, it should be removed.
We figured it out, it is called adventitious root or "water root". It's caused from high humidity or rainfall. It will dry up and fall of an a week or so, and won't cause any damage to the tree.
Glad you found that out. We see adventitious roots on tomatoes often but I had never seen them on Willow.