I have off white rubbery growths on the bark of my willow tree.
It has happened in the last few weeks with the wet weather.
Some fungus similar to shelf mushrooms. The bark looks abnormal as if damage has been developing over time. Likely the fungus has "roots" into the wood and the trees days are numbered. However, it may live for years. I had an oak with a shelf mushroom that gradually died over 20 years. I doubt there is any way to stop it. Plant another tree nearby if you want a tree in that location.