The original Python implementation is called CPython. pypy is another implementation of the Python language. Both exists in versions compatible with the Python 2.7 and Python 3 versions of the Python language. Each combination of Python implementation and Python version installs packages into different locations on the file system. On a system with multiple implementations and/or versions installed you’ll usually have one pip command for each.
Thus, when you use pypy’s pip command to install the extension you install it in a location entirely unknown to the Python implementation that runs Mopidy.
Assuming you’re running Raspbian on your system, you can install pip for Python 2.7 with:
sudo apt-get install python-pip
This will give you two equivalent pip commands:
/usr/bin/pip
/usr/bin/pip2
If you use one of these to install the extension, Mopidy should find it when it is restarted.