I installed https://github.com/forslund/mopidy_skill, it seems it tries to work but when I say "spotify rock music" it tries to run Wolfram skill. How can I differentiate those skills?
So if it's running Wolfram, it means that it's missing your skill. There's probably an error in the instantiation. You can check the logs in /var/log/mycroft and see if you're getting an error message.