I just got my mice today, but when I opened their little carriage thing to get them into they're new cage, the floor was covered in squishy, green poop. They're both from Petco, but look perfect,y healthy. Are they sick or is it just something they eat? I feed them oxford brand food, and some of the less healthy mouse mix with sunflower seeds, nuts, and other grains. Should I stop feeding them that brand or should I just return the mice. I don't want to wake up one morning just to find them dead if they really are sick.