Any change in diet can change the color of their poop. I think the mustard greens is what made Sienna's green, but if the color is new I would probably say it's the new food.
Please throw the Nature Zone Bearded Dragon Bites in the bin, they are worthless nutritionally because they are 94% moisture and only 3% protein (anyone's guess where the protein is sources). They are a total waste of money and not even suitable to use as treats for ANY REPTILE.