i never get the popup about usb connection when i connect my moto g 3rd generation android 6.0 to pc. due to it, I am not able to connect my phone to any pc or laptop. it is in the default charging mode.
Marshmallow, on the Moto G 2015 at least, does not give a pop-up like Lollipop did, you must manually grab the pull-down menu and select USB Charge Only and switch it to MPT each connection.
There is an option in Developer Options to set the default USB operation, but the setting seems to have no effect on operation. As of the current MM release it isn't known if this a bug or as designed because Moto hasn't made an official comment on it.