Could you please check if you have SSH access allowed on the WAN zone? Navigate to Administration > Device Access > Admin services.
If you would like to block the IP address, you could do it with Local ACL rule. Please follow this document for more info: Add Local Service ACL Exception Rule.
You are getting alerts from the firewall because SSH is allowed on the WAN zone. That means this service is available on the internet.
If you need to access your firewall via SSH from the internet, consider creating Local ACL exception with source public IP and remove SSH access from the WAN zone.
