Obstruction
Judah did this every once in a while a couple months after we got him, and I think I have posted this before but here's what it turned out to be: Dryer sheets and/or small socks. It turns out that when we first got him we got him a plush toy, and he had an accident in his crate, and we had to wash it. Well it was one of his favorite toys and it smelled like dryer sheets after we washed it, so naturally...
It took a while to figure out why he was throwing up occassionally (also a mucusy white and yellow mixture) but then I noticed something wierd in his poop; Sure enough, it was a dryer sheet. We have been much more careful about making sure none fall on the ground in the dryer area, but a couple of weeks ago he substituted one of my thin black dress socks for the dryer sheet, and that night was up again with the mucusy vomit, and wouldn't eat much the next day, then BLAM! he yacks up a little black sock. So whoever suggested obstruction might be on to something. By the way the throwup looked sort of like what a person would cough up if they had an sinus/lung infection but only a lot foamier. Hopefully you'll figure it out.