As soon as you hit Revered with Golden Lotus, a new quest line spawns for you; this causes certain parts of the Vale of Eternal Blossoms go into a different phase and has the unfortunate side effect of making the target mob for this quest completely unreachable.
The only way to correct this issue is to return to the Golden Pagoda (the place underneath the big panda statues with crossed spears where you always start the daily quests for this zone) and complete the newly spawned line that starts with this quest:
Battle Spear of the Thunder King
Once this line is completed, the zone will return to normal phasing and you will once again be able to complete this quest.
This is as of patch 5.0.5; the issue may be patched by the time you read this comment and this workaround may no longer be applicable.