I can't remember if it was here on DG, or another forum, that I saw(I think it was:) Fred comment on a brug that looked just like that.
He said it was a fungus if I am remembering correctly. And why was this important to me? I had the exact same thing
happening to one of mine. Maybe someone else knows better, or has a link to that thread. I'll keep looking for you.
Good luck too.