The process is not already running. I checked. I went into config and disabled HTTP, but then got the same error when trying to start mpd server.
ERROR 2015-04-04 03:31:11,202 [2765:MainThread] mopidy.commands
Frontend (MpdFrontend) initialization error: MPD server startup failed: [Errno 99] Cannot assign requested address.
When I changed to hostname = 127.0.0.1 for mpd in mopidy.conf, then the frontend started ok.
2015-04-04 15:32:15,824 INFO [2466:MainThread] mopidy.commands: Starting Mopidy frontends: MpdFrontend
2015-04-04 15:32:15,847 INFO [2466:MainThread] mopidy.mpd.actor: MPD server running at [::ffff:127.0.0.1]:6600