What does "Unique Equipped" mean?
It means you can only equip one of the item in question. This is mainly relevant for rings and weapons, but is probably just automatically applied to other legendaries.
Usually unique equipped means that only one of the items may be equipped at the same time.
E.g. if you had 2 of the same ring and it was unique equipped you would only be able to wear one of them.
But as this is a spirit stone which is equipped in the head slot it doesn't make any sense.