Stackdriver alerts don't close automatically

Closing incidents Incidents are closed automatically; you can't close an incident. An incident is closed when the system observed that the condition is no longer being met or when 7 days have passed without an observation that the condition is still being met.

Even though, you can "Aknowledge" them, which will basically let the system know that your are aware of it and will allow the alert to keep working as expected, otherwise, since the incident is opened, no new alert will be triggered in the meanwhile.

To mark an incident as acknowledged, do the following:

In the Incidents pane of the Alerting dashboard, click See all incidents. On the Incidents page, find the incident that you want to acknowledge, and then do one of the following:

Click (3 vertical dots) More options and then select Acknowledge. Open the details page for the incident and then click Acknowledge incident.

After Acknowledge the incident, open the still alerted incident and you should see a "Silence Incident" option, when you hit it, it will change the incident status to Closed.