Ficus ‘Green Island’ is one of the cultivars of Ficus microcarpa. Its main distinguishing characteristics are thick, rubbery leaves that are nearly round. This round leaf character is not unique to ‘Green Island’ but can be found in seedling Ficus microcarpa.
I have one seedling with very round leaves that is not ‘Green Island’ since it is not grown from cuttings of ‘Green Island’ but is genetically distinct as it is seed grown.
One can only call a plant a varietal name if the parent from which the cutting is obtained is the specific variety.