It might be something your dog ate causing this. It might also be an overgrowth of a bacteria that is normally present in your dog's digestive system called clostridium difficile or c-diff for short. This will cause bloody diarrhea, make sure your dog gets enough to drink, and take him to the vet if it continues as you don't want your dog getting dehydrated and he may need antibiotics.
I took my dog on a walk this morning and when he went to the bathroom his poop was green and it had blood in it. He had the diarrhea just like water. What could this be?
The main thing you have to watch for is parvo. This involves green diarrhea with blood, but it does have a very foul smell. If this happened with your dog, I would get him to the vet as soon as possible.Has your dog been ill in any other way? Maybe a bit lethargic and possibly losing weight? It would help to know the other symptoms as there are a few things that this could be; some easily treatable but other illnesses are more serious.
Other things that cause green diarrhea include eating grass and not digesting it properly, eating rodent poison, or eating a rodent that has eaten rodent poison.
The worrying thing is that there is blood present. This is definitely something you need to get checked by a vet. If at all possible (and I know this sounds both difficult and gross) try to get a sample of your dog's stool or diarrhea. Your vet will be able to test it for all sorts of problems which will make treating your dog a lot easier.
I really hope he gets better very soon, let me know what the vet says.