Hi,
I think that the relation between mind and Moon is of correspondence.
This is a correspondence between two realities, one located within microcosm (the mind), and the other located within the macrocosm (the Moon). I this is why the mind is the microcosmic counterpart of the Moon.
I am thinking of the ancient Rishis exploring their microcosm in a practical manner, and finding these correspondences, which they expressed in written form afterwards.
Such a correspondence is to be found be mere spiritual intuition, and can be proved afterwards by using analogies.
So, I think that mind and Moon are in correspondence, among others, because of the following aspects:
1.
reflection;
2.
fluctuation (as Matthew nicely pointed out) - but I think that mind is subjected to more influences than the Moon is, so that its state of fluctuation is more complex that the one of the Moon; we just have to think to the five tattvas, to the three gunas, etc.;
3.
seen / unseen - just like the Moon, which has a hidden face, the mind a ”hidden” part, which is the subconscious.
I am sure that more analogies or similitudes can be found, in order to prove that the mind and Moon are in correspondence.