I am trying to replace my idler pulley but am unable to take off the retainer clip. Do I need a special tool to do so? Is there a special technique for removing it??
Looks like it is held on with a circlip (sirclip) also called a snap ring.
It requires a special set of pliers, they have small prongs that fit into the clips holes. When you squeeze the handle they expand the ring so it will come off.
Note there are different size prongs/holes so if you are getting a set make sure it will fit the ring on the idler.
I have a pliers set that was not expensive and has a handle and a number of different heads for small and medium circlips. Also lets you remove/install expansion and contraction type rings.