From the toast message,
and comparing to a source code, it seems that the user is using Simple Last.fm Scrobbler.
The app also supports many music players, such as
Supported apps: Android Music Player, HTC Hero/Sense Music Player, Google Music, PlayerPro, Meridian Player, MixZing, Archos Music Player, Rhapsody Android, Vanilla Music Player, bTunes, ³ (cubed), WIMP, Zimly, Just Playlists, PowerAMP, Winamp, myTouch 4G, RockOn, jukefox, DAAP Media Player, Folder Player, GoneMAD Music Player, MIUI Music Player, ServeStream, Default Sony Ericsson/Xperia Music Player, Neutron Music Player, Music2Dance2, Rdio, DeaDBeeF, Music Pump XBMC Remote (Beta), JRTStudio Android Music Player, Music Folder Player, Apollo, LG Music Player, Now Playing, Shuttle Music Player, My Cloud Player, SoundSeeder, Amazon Music, more...
Uninstalling the app will remove the toast message when playing musics with listed players.