It is definitely a sun. And you can see the shadow under it as if it mounted, so it isn't a mosaic. The drop shadow from the overlay makes it a little hard to guage it's true dimensionality. I haven't been in the newly designed Land pavilion yet, so it might be from in there.
A bit of trivia: note the indicator labeled MAPO. This is short for Mary Poppins, the film that generated the revenue that WED (Walter Elias Disney) Enterprises used to develop the monorail system in use at Disneyland and Walt Disney World. It's a proximity system that alerts drivers when they are getting close to another train and will automatically stop the train if the driver doesn't respond.