Did you refuse to get George the Drudanae herb at the start of the game? That will break the murder quest, even if you later bring him his fix.
If not, just check back with Mardaneous; after some other change or quest update, or something, the proper speech option may get triggered. I defeated the vampire but did not go back to Aleroth until after the castle, when doing the council quests. During the conversation, I talked about George first and then recruited Mardaneous, and didn't have any problems. Some people have been able to finish the murder quest after he moves, so maybe the speech option will get triggered then.
