The map for the Inverted Arbor in the Abyss 2nd Layer has a side quest near the entrance to the Corpse-Weeper Den. I've explored both the upper catwalks and the lower islands near the spot indicated by the map, but the side quest won't activate.

How do I activate the side quest in the Inverted Arbor?

[![Inverted Arbor side quest marker][1]][1]


  [1]: https://i.sstatic.net/7GsAS.jpg