Free the Green Sage was a mission that Jak and Daxter completed to obtain a power cell in The Precursor Legacy. One of four sages that was kidnapped by Gol and Maia and brought to their citadel until Jak freed him from their captivity in this mission.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Set Me Free Already!", completing this mission will reward you with a bronze trophy.
Using high jumps these platforms can be traversed safely. After having done so you'll find another short bit with the broken precursor platforms. Make your way up to and turn around to face the last obstacle leading to the top of the citadel.
There are four large rings here which continually spin around with several 'arms' poking out at set intervals with metal platforms at the end. You'll have to jump on to these one by one (you won't fall off the slanted one) to finally reach the wooden walkway at the top. Here you'll just have to walk around it (going counter-clockwise) until you find the last generator. Destroy it to find the last power cell and free Samos Hagai. He will proceed to use his green eco powers like the other sages.