Grease as it gets older breaks down to a more liquid type substance. there is no gasket or oil seal between the transmission and the attachment hub, thereby allowing liquid to pass through.
In my shop we dismantle the entire transmission to empty the cavity and clean out all the old grease before rebuilding and adding new grease. if you don’t have the ability to do so, you can add fresh grease to try to correct the consistency of what is in the chamber already to prevent leaks
If you are unfamiliar with the mechanics of this unit I would recommend you taking it to a service center