If you receive the message "You see nothing out of the ordinary," simply wait for the leeches' bodies to vanish. Then you can get the coral by checking the ???. Not everyone has this issue.
If more than one party member needs this fight, only one person needs to pop the three leeches. After the last leech fades away, have a different party member who needs the item click the ???, then have the player who spawned the leeches click it afterwards. Otherwise the player who did not spawn the leeches will be unable to obtain the Strangely Shaped Coral.
Return the dagger to Ensetsu for a cutscene that finishes the quest.
The Korroloka Leeches immediately attack the player who examines the ???. If this player is low-leveled, he/she may die from this. It is 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 the leeches' attention.
It is not necessary to kill all three leeches. If you hold one leech without aggroing or linking the other two, you can wait for the two passive leeches to despawn. Then, you can kill the leech you were holding and receive credit for the fight. The game only checks that the last remaining leech is defeated. Note that spawning the leeches and letting all three despawn does not allow you to get the key item.