First, let me tell you more of the task you must complete. The druids in Darnassus use the water of the moonwells of Teldrassil, and their moonwell must be replenished from time to time. Using these specially crafted phials, you can collect the water of the moonwells.
Take this vessel to the moonwell outside of Starbreeze Village to the east, and fill it with some of its waters, then return to me. When you have completed your task, I shall continue the story where Tenaron left off...
Hello again, <name>. Have you completed your task?
After the Battle of Mount Hyjal, we were without direction. Nordrassil smoked from the fire it unleashed, and our immortality--the very essence of our beings!--was lost.
The moonwell [63.3, 58.1] is just outside Starbreeze Village, on the side closest to the road. To get to Starbreeze, follow the road east from Dolanaar until it branches off, and continue on the east branch. You should arrive at the moonwell before actually entering the village. Right click the vial in your inventory to fill it while standing in the water, then return to Corithras in Dolanaar to complete the quest.