My new gerbil has green, mushy poop and a red bottom. Is it serious?

Hi there! While diarrhea in new gerbils is not entirely unusual (as stress and food changes can cause soft stool), I am concerned about the combination of continued diarrhea and the green coloration. Unless you have been feeding Peach a large amount of green-colored food, green coloration usually indicates a large amount of bile released in the intestines, and this can indicate a disease process. A common diarrheal disease of Gerbils is called Tyzzer's disease, and it can cause fatal dehydration if not treated. I'm glad to hear that Peach is still eating, drinking, and acting normal, but if the diarrhea continues or if his attitude changes, I strongly suggest taking him to the vet as he may require antibiotic treatment and fluid replacement therapy. This condition may also be contagious, so keep a close eye on the other gerbils for similar symptoms. Hopefully, the diarrhea will abate on its own, but if it doesn't after a day or so, please consider contacting a veterinarian. Best of luck to you and your new friend, Peach!