It is not an icon for cluster. This icon is shown for servers, which have remote agent utility configured to publish to this media server. In order to check, publishing, go to remote agent utility on the target server and go to publishing tab and check if publishing for media server is enabled. Publishing is a feature by which it becomes easier for media server to detect the target servers in backup. These servers should also be visible under favourite resources, in selection list.
Please mark it a solution, if this is useful.
Thanks