Almost the same exact thing is happening to me, and I am currently on day 13 of a 20 day water fast. The nausea and throwing up started on day 10 and went on for a couple of days. Same green stuff you're describing. It went away for a while, but it's back today and I can barely drink water -- it really unsettles my stomach. I know I was freaking out about it too, until Andrew explained it :) I hope you feel better soon!