I wanted to put in an additional wrinkle to this issue.
In addition to logging out and logging back in causing the moon pieces to vanish, I also got some of the moon pieces disappearing when I left the room and returned, even without logging out. I did that because a couple of people in my party were having a conversation when I first entered the room, talking about how this was described in "that journal" or something like that. I don't remember ever reading anything of the sort. Of course, I'm so out of order on this game and had to roll back about two hours of game play, but that's a different issue. I left the room because I thought it might have been in one of the journals or books that I left behind in the goblin priestess' bedchamber. It wasn't there, or at least the clue wasn't in anything I remember reading in there.
Additionally, When I returned to the room, Pale Elf guy succeeded at a perception check that the rest of the party failed, noting "something interesting", but I never saw what he thought was so cool. Based on reading above, I'm assuming now that he found/saw the lever to open the door, which I suspect only gets a roll to appear if you solved the puzzle correctly. But I have no clue if I did solve the puzzle correctly, and I can't find the lever now. Also, after saving and returning to the room, as all others above are posting, the moon phase sigils (sp?) have completely vanished, and I have no clue what is up.
***SPOILER WARNING*** from this point.
Didn't realize it to be a path to the Underdark, but since the Drow and the goblins are working together, I'm not surprised to read that. I expected it to be a path to an underground prison where Haslin (the real ArchDruid) is being held.