It was the corruption of the initial "moonlight" glow when it was Minas Ithil, the original name.
I found the following regarding the glow:
"Upon the further side, some way within the valley's arms high on a rocky seat upon the black knees of the Ephel D?ath, stood the walls and tower of Minas Morgul. All was dark about it, earth and sky, but it was lit with light. Not the imprisoned moonlight welling through the marble walls of Minas Ithil long ago, Tower of the Moon, fair and radiant in the hollow of the hills. Paler indeed than the moon ailing in some slow eclipse was the light of it now, wavering and blowing like a noisome exhalation of decay, a corpse-light, a light that illuminated nothing. In the walls and tower windows showed, like countless black holes looking inward into emptiness; but the topmost course of the tower revolved slowly, first one way and then another, a huge ghostly head leering into the night."
It isn't described as green but an "exhalation of decay", which Peter Jackson interpreted as a evil green glow.
I can't wait for mine to eventually ship and arrive. The LOTR Environments are my main collectible line and this was a must-have for years.
By the way, I have a custom Sammath Naur piece that I treasure.