You mean yellow liquid or yellow solid or a combination?
It is just excess bile. It happens when you aren’t used to fasting. Your gallbladder keeps pumping out bile into your intestines in anticipation of food coming through, but since it doesn’t come you just explosively shoot it out. It doesn’t happen once you become more adapted.
It’s normal. Your body is expelling leftover shit from your digestive tract. It eventually stops. It doesn’t even happen to me anymore, even when I stop fasting for weeks. It will float, stink, and be mucous like. It’s normal afaict.