This is a basic Alexa skill that allows control of an MPD server. The MPD server must have a publicly accessible URL. This should be easily achievable with port forwarding and dynamic DNS or a fixed IP.
I have not tested with Mopidy but in theory it should work.
It uses the mpd-api module
It just has basic features as that is all I need (I mostly use ncmpcpp) but should serve as a good basis to extend into an all singing all dancing MPD control.
FYI I use MPD with Snapcast for multi room audio (as Alexa's sucks).
You should be able to import this as an Alexa hosted skill directly from this git repo, I have not tried this my self though. Instructions here (let me know how you get on).
https://developer.amazon.com/en-US/docs/alexa/hosted-skills/alexa-hosted-skills-git-import.html
Enjoy!