According to this thread, the trigger for spawning the myconid elder is a small lootable pile. But I have not been able to find it.
During my first encounter of the dying myconid, it did not have anything for loot after it died. And after I came back to that location, the dead myconid is already gone.
I used one of my previous saves to go to that location again, and confirmed that there is nothing for me to loot after the dying myconid died. This issue seems to be a bug of v18.104.22.168. I had to use a console command to spawn the myconid elder in order to complete the quest.