You can see this green Datacron above a waterfall in a cave near the Jedi Temple of Tython.
You can get the datacron by carefully jumping up the waterfall cave entrance where you can see the datacron, but there is also a secret entrance further on up the trail in the river that doesn’t require any jumping.
Quick Travel to Elarian Trail or Taxi to Forward Camp Speeder
The closest Republic quick travel point is the Elarian Trail quick travel point (in the south of the map) and the closest taxi point is the Forward Camp Speeder speeder point (in the most south taxi point on the map).
Follow the River
From the Forward Camp, head up the stairs, following the path to the east towards Fleshraider Territory. Once you find the river nearby, follow the river north until you find the secret cave entrance. The datacron is at the end of the tunnel.