I believe the only documentation re. logging to SD Card is via the blog: https://moosh.im/2015/03/new-firmware-update-release/

The last step on those instructions are to disconnect from the meter – not just leave the app (which will be running in the background, on Android), but actually disconnect via the ‘plug’ icon on the list of mooshimeters. One way to tell if it’s logging is that the LED blinks every time a sample is written.