The phrase "green slime" refers to a substance that is slimy and green in color. It is often used to describe something unpleasant or disgusting. Full definition
When you kick them, they start to expand and grow, like a water balloon filled with green slime. (inc.com)
A very contagious intestinal virus is responsible for a disease called «green slime diarrhea» for lack of a better name. (hallettvet.com)
That said, if one of the new ferrets has the viral diarrhea (commonly called «green slime disease»), it can be very hard and sometimes lethal to older ferrets. (drexotic.com)