In order to do this quest you must have unlocked the Murlock (Mrrl) in Nazjatar.
How to unlock Mrrl vendor
To get access to his vendor you must complete the quest A safer Place. This quest is to escort him to your base. You find the quest at 48.43.
How to get 5 Dense Rocks
- Buy 105 Flatulent Fish from Grrmrlg (Shift + Right click and type the amount)
- Buy 15 Jar of Fish Faces from Hurlgrl
- Buy 15 Bag of Who-Knows-What from Flrgrrl
- Buy 5 Particularly Dense Rock from Mrrglrl