Suzanne Cianciulli, a veterinarian technician who manages the nearby Junaluska Animal Hospital, gave a very simple explanation: poop.

“Each puppy is located within their own sac in the uterus, and the sac is full of fluid,” Cianciulli reported. “If they have meconium in that sac, it will actually stain their fur.”

First poop, now “meconium?” According to CNN: “Meconium is an infant mammal’s first stool.” So it’s the stool a parent would save in a special locket and cherish forever. “The color is temporary and goes away as the puppy gets bathed or cleaned by the mother.”

So basically, Hulk had a green poop in the womb, it died his fur, and now poor Gypsy has to lick it off till he turns white … Charming!

While this is a pretty rare phenomenon, there have been multiple Hulks over the years. Some dogs are Edward Norton-types, some are Lou Ferrigno-ish, and others are more Mark Ruffalo-esque, but none have suffered major health consequences. That only happens if a puppy “ingests the meconium or spends too much time in it.”

Obviously, Hulk has better things to do. Per CNN, “he and his seven siblings are doing well, and will be adopted by loving families once they’re old enough.”