You'll need at least +10,000 faction with Nae'Caelona to see the plant/get the quest.
You have 20 minutes to:
Obtain water from the base of the fall at the Waters of Nye'Caelona (location update)
Uncover and kill whoever's poisoning the water. kill 15 a vrassath saboteurs (they are invisible, all around the waterfall area - look for flat areas.)
Collect 100 crystal dusts from geomantic extrusions on the trees (big purple rocks on tree trucks in the Garden area). You repeatedly "release" the dust from rocky extrusions and click on the floating balls that result to collect. The dusts can be collected while flying.
Go to the top of the falls to release the crystal dust
Go to the base of the falls to collect some water
Go to Nye'Caelona and give water to 10 a dying plants found in the elvin city area.