It's not, I've seen it in at least a half a dozen other photos in different colour bands.
This is the Jet Propulsion Lab's release on this phenomenon.
Hexagons are a pretty "parsimonious" shape -- they appear in nature a lot, as they provide the optimal "packing" arrangement.
I wouldn't be at all surprised to learn that we're seeing the intersection between a series of longitudinally distributed high-latitude cyclonic disturbances caused by something like shear effects between atmostpheric layers, or something like that.
Wow. Cassini has yielded SO much fascinating information.