The path
“/home/pi/Music/Graceland 25th Anniversary Edition/05 - Diamonds on the Soles of Her Shoes [Clean].mp3” doesn’t mean anything to Mopidy. You need to give it a path (URI) it can understand, which may or may not have any relation to the path on your filesystem (for Mopidy-Local it currently does but that could change). Use mpc ls to browse the virtual filesystem exposed by Mopidy and find the URI for the track you want, for example:
nick@xps:~$ mpc ls
Files
Local media
SoundCloud
TuneIn
nick@xps:~$ mpc ls "Local media"
Local media/Albums
Local media/Artists
Local media/Composers
Local media/Genres
Local media/Performers
Local media/Release Years
Local media/Tracks
Local media/Last Week's Updates
Local media/Last Month's Updates
nick@xps:~$ mpc ls "Local media/Tracks"
local:track:Music/song.mp3
mpc add "local:track:Music/song.mp3"
mpd and Mopidy have different URI schemes and expose different virtual filesystems for the same media collection. The path for a given track may therefore also be different and should not be considered interchangeable between these two different programs.
Mopidy-File (bundled with Mopidy) has a different approach and the URIs exposed will always be related to the path on your filesystem, but you still need to specify the file:// prefix so Mopidy can understand e.g.
nick@xps:~$ mpc ls "Files"
Files/Music
Files/Home
nick@xps:~$ mpc ls "Files/Music"
file:///home/nick/Music/song.mp3