Yoshi Juice is juice that Yoshis shoot in Super Mario Sunshine, out of their mouths. The color of the juice depends on the color of the Yoshi. The juice is not different from the water FLUDD shoots, however the juice can wash away Sponge Goop, while water can't.
It is unknown why the Yoshi Juice doesn't appear in other games, however it is believed that only the Yoshies in Isle Delfino can shoot juice.