Motorola g3 and unreadable SD Card
The issue here is that Marshmallow and newer Android external storage is encrypted when it is adopted by the system as internal storage. 

You need the crypto key from your device to mount the volume, does your phone have an unlocked bootloader and/or are you rooted? If not, you are not going to be able to access the key and decrypt the volume.

Similar questions which may be useful as reference: Moto G3 (android marshmallow) not detecting SD card anymore (solution?) and Recovering data from corrupted SD card formatted as internal storage (Marshmallow) 

Remember that this is not unique to the Moto G 2015, most Android devices with Marshmallow and newer have this capability (and issues). Honestly, it best to not use adopted storage if at all possible.