The green check mark is for the vCenter Proxy Service. In this case you have two cells and at least one vCenter. If you shut down the first cell, the vCenter proxy will migrate to cell number 2.
The vCenter Proxy is what sends commands to a specific vCenter.
e.g. when load balanced your HTTP session might go to cell #2. you request to copy a vApp from Catalog to My Cloud. Cell 2 queues the request in the database. Cell 1 sees a new task to be done, and performs the copy commands. Cell 2 will refresh the website information, and you see tasks being performed.
If you want to continue to have failover, you should have N+1 cells. Where N is the number of vCenters attached to the vCloud Director installation.