Just so you know if your thinking about switching to sand, you'll kill your dragons or at least put them in harms way. Sand is down right unnatural and its a shame people keep using it for beardies when they dont even live on it in the wild.
This is a few places in Alice Springs where bearded dragons are commonly found. As you see the environment they live in is mostly dirt, rock, minerals, clay, shrubs, trees ect. It may have a bit of sand in some parts but for the most part there barely is any at all.
Just so you know its not just impaction that is a risk, its prolapses, fungal infections, eye infections, scale rot, joint issues, and it also provides a bacteria breeding ground so no matter how much you clean the out poop and old food it'll always be dirty.
Personally I dont see anything wrong with his stomach but if there is green marks I'm guessing its probably because of the substrate.
Which if it is then I'd suggest paper towels, tile or non adhesive shelf liner.