If I have a ESXi server with a green circle in map view visualization, what does it mean?

A.
The host currently has free resources.

B.
The host is down

C.
The host doesn’t have free resources

D.
The host is not in a cluster

Explanation:
A host that is compatible for vMotion migration. The color of the circle varies in intensity based on the load of the current host. Heavily used hosts are pale; low-load hosts are saturated green.

http://pubs.vmware.com/vsphere-50/index.jsp?topic=/com.vmware.vsphere.vcenterhost.doc_50/GUID-BBA5CE4B-3E12-4B65-ADE5-265477FAD22D.html