When I created a Cloud SQL instance (MySQL), a new service account was created and showing up on the instance overview page. There was no mentioning of that or of its usage/purpose in the class or lab. The service account didn’t show up on the "IAM & Admin" Service Accounts page, so there is no way to find out its detail.
Cloud SQL has to run with a service account as its identity, which is the service account you’ll see on the instance overview page in the dashboard. However, this is a google-managed service account (as opposed to a customer-managed service account that you might create yourself), so you cannot see it or manage it under IAM & Admin.
Hope this helps!