In the majority of cases where a dog does not eat especially after a long road trip, at least to them, green feces is generally just the result of flushing out potential bacterium in their system. You also have to take into account that some dogs get very upset by riding in car, especially if it is new to them for long periods at a time. However, no dog, regardless of their breed or overall health, is totally free of bacteria, so this should not be a real concern.
However, with that said, if it continues, your dog may have an intestinal infection as the result of a parasite infection from some type of worm which is also quite common, especially in large breed puppies. The warning sign here is not diarrhea, as it is the complete opposite, which is constipation and your dog straining to defecate, and when they do; their feces will be green in color. It could also be the result of your dog eating grass as well as some type of an allergy to the food that you are feeding them, and the period between not eating again is flushing it out.
The key is to watch to see if it continues, and if it does, you will need to have your vet do a stool examination for some type of worm infection. If it goes away, it should not be an issue unless it is an allergy, in which case it will resurface at some point and then you will know that it is a reaction to their food.
It could be a possible infection that is washing out or caused from its food. The type of food may cause it.
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