The latest update for the Android Google Music App breaks the compatibility with SoundSeeder.
SoundSeeders Google Music support is based on the Chromecast API. Google is now blocking third party apps in the Google Music Player to use this API.
Last known working version is v5.5.1514O.1185867
You can uninstall the latest update and install this version manually to use SoundSeeder with Google Music again. Download GM v5.5