What you are seeing is a Canvas. Canvases are immersive visual layers replacing the cover art on the Now Playing View screen. Currently it's only available for mobile devices. The content will be scaled for screens in portrait mode.
Every Canvas is created by artists for their own music. It will start playback by default whenever you play a song that contains a Canvas.
Below you can see two examples of tracks that have a Canvas.
Canvases are optimized to use very little data and battery. You can toggle between having Canvases enabled by going to the Settings.
Home > Settings > Canvas
Home > Settings > Playback > Canvas
New Canvases are being added every day so look out for visual treats in the future!