_**Hex**_ is a spell, and I am sure you know what _**cast a spell**_ means.
As for _**watermark**_ , it means
> a design which is put into paper when it is made, and which you can only see if you hold the paper up to the light. Banknotes often have a watermark, to make them harder to copy illegally.(Collins)
Now, if you look at the moon it does seem it is watermarked with a mysterious design. However, one can't be sure if the author of this verse used the moon as a watermark, as a design to imprint on something as a logo, like:
![enter image description here](
or if he refers metaphorically to this watermarked-like complexion of the moon:
![enter image description here](