You must wait for all 3 Leeches to 'vanish' or else you'll just get a message of "You see nothing out of the ordinary." Once their corpses vanish, you get the coral checking the ???.
I did not find this to be an issue, I checked it when there was still one leech dead on the ground and got the item.
If more than 1 member of the party need to complete this portion of the quest, only one person needs to pop the 3 leeches. After the last leech fades off have a different party member who needs the item click the ???, then have the player who spawned the leeches click it afterwords. Otherwise the player who spawned the leeches will be unable to obtain the Strangely Shaped Coral (key item).
It is not necessary to kill all three, as you can kill off one if you manage to not aggro the other two and they de-pop
Return the coral to Ensetsu who will ask you to take it to Norg.
Return the dagger to Ensetsu and watch a cutscene to finish the quest.
Notes
The Korroloka Leeches will immediately attack the player who examines the ???. If this player is lower level, and may die from this, it may be a good idea to have that player use Sneak (item/spell/ability) to prevent this, or have another player use an AOE attack to quickly attract their attention.
If the player who examines the ??? dies, as long as all the Korroloka Leeches are killed before they are raised, they can still obtain the Strangely Shaped Coral by checking the ???.
Can be beat by a full party of level 35+, and by many jobs solo at 50+.