It looks like someone might have named this alert wrong, as the content of the message suggests it is an "Full access permissions granted" alert. The name can be anything, and it does not relate to the actual alert configuration. Perhaps someone copied an existing alert to be used as template? Anyway, check it under the Compliance center: https://compliance.microsoft.com/alertpoliciesv2
As to "NT Authority\SYSTEM", this indicates that the action was performed by a background process, and you can safely ignore it as a "system" event.