Mornath's Custodian is friendly and cannot be killed. This npc is mostly likely causing the quest to be bugged because the custodian isn't killed and the quest stage isn't completed.
The issue below will most likely be eliminated after fixing the one above.
According to Wowhead comments: "I couldn't click the chest despite reducing Mornath's HP to 1. When I killed his "pet" before killing him it finally turned clickable."
Links:
https://www.wowhead.com/quest=41466/estate-jewelry-haute-claw-ture
What is happening:
The issue below will most likely be eliminated after fixing the one above.
What should happen:
According to Wowhead comments: "I couldn't click the chest despite reducing Mornath's HP to 1. When I killed his "pet" before killing him it finally turned clickable."