Green blood is the blood of aliens and human/alien hybrids, which due to the toxic and acidic nature, human exposure to it is mostly fatal and corrosive. (TXF: "The Erlenmeyer Flask", "Colony", "End Game", "Nisei", "Musings of a Cigarette Smoking Man", "The Unnatural", "Requiem").The green blood contains a retrovirus which turn the human blood into jelly-like substance. but the effects of retrovirus can be neutralized by extreme cold. (TXF: "End Game", "Emily")