On newer Android phones, especially Samsung, a small notification tile can appear at the top of the screen showing the currently playing song's artist and title. Here's how to hide it.
Samsung (One UI 7 / 8): go to Settings → Lock screen and AOD → Live notifications and turn Media player off. On some devices this option is not in the menu directly; use the search field in Settings and search for 'Live notifications'. Note that this also removes media controls from the lock screen, but during QRSong! gameplay you don't want those visible anyway.
Stock Android / Pixel: long-press the Spotify notification in the notification shade, tap the gear icon, and disable the 'Now playing' or 'Media playback' notification channel. As a stronger fallback, go to Settings → Notifications → Notifications on lock screen → Don't show any notifications to hide all lock-screen notifications.
No-settings alternative: place the phone face-down during play, or play through an external speaker.