Think I have figured out what causes this. It only happens on old rotting Alder branches and only when the temperature reaches a curtain degree. What I think happens is the branch is giving off a gas as it rots and the gas freeze causing the white bloom. It is all ice and quickly melts as the temperature rises.
Icy Branch
Icy Branch
Three Scores and 14 years old and still going strong.
Re: Icy Branch
Good theory. Sure you are correct too.