Did you go back and talk to Angaredhel? There are several conversations you need to have.
http://uesp.net/wiki/Morrowind:Gateway_Ghost
Yeah I did--revealed the name of the culprit and got the mage ring.
I suppose as a backup plan I could just click on the next iteration of the ghost and type "disable" in the console, but if the quest is bugged I'd like to try and figure out what's causing it. The only mods I have that might modify it are Piratelord's Creatures.