For the intro level, green can apply to either rider or horse. Some people use them to introduce a young horse to showing, and some use them to introduce new riders on experienced horses to it.
For dressage, what they are looking for at that level is just your bare bones acceptance of the bridle, a nice relaxed horse traveling in a consistent rhythm. Your basic bottom of the training scale stuff. If you can show some impulsion and elastic contact, you'll come out of test a happy camper. Things like bracing, stiffness, head tossing, tail swishing will get you poor marks.
At the shows I did, the stadium round was 9 fences, very straight forward, no combinations. The whole thing is geared toward getting rider & horse confidence up.
You just have to see your distance...you don't have to like it.