It's the sunlight reflecting off the top of the greenhouse.
--- Update ---
Here are my two theories for how the projectors could be positioned.
On the right is my rudimentary sketch of sort of what a flying bench on an arm would look like (severely simplified so as not to actually look remotely like what the real robotic vehicles might look like, in compliance with forum policy :smiley: ). For option 1 for the projectors, the projector would be embedded in the base of the vehicle, so as to travel at a constant speed and the same rate as the carousel screen. The bench would still have a freedom of movement. For option 2, the projector is attached to the top of the screen, basically out of view. This would probably make it easier to sync the projector and screen.
I might also add that with regard to the sound: While it's true big theater screens have speakers behind the screen, with a situation like the domes, with the "audience" being only 4 people, stereo speakers placed on each side of the screen will be adequate to simulate a phantom center channel. In big theaters, the reason they put the speakers behind the screen is that 1) the speakers have to be HUGE to fill the large theater 2) it saves real estate for the screen, and 3) for audience on the sides of the theater, you need a center speaker or else the on screen action and dialogue will sound as though it's coming from the side rather than the center.
Additionally, due to the parabolic shape of the screen, they can actually "project" sound into the dome, much the way they do with the video. The sound will bounce off the screen back towards the ride vehicle, making it seem as though the sounds are coming from the screen itself. They can easily design the screen with this in mind (making the screen material very acoustically reflective) and hence the application of screen goo wouldn't disrupt the sound because the screens wouldn't need to have holes... not to mention that if you're going to be that close to a screen, essentially only a few feet from it, you can't really have holes in it since they would be visible to the naked eye, unlike large theaters where you're usually a minimum of 20 or more feet away, in which case you can't see the holes. For the best visual quality in an "up close" situation, the screen should be flawless for the best illusion of "being there."