Feather Step: "For the duration of this spell, the subject ignores the adverse movement effects of difficult terrain, and can even take 5-foot steps in difficult terrain."
Grease: A grease spell covers a solid surface with a layer of slippery grease. Any creature in the area when the spell is cast must make a successful Reflex save or fall. A creature can walk within or through the area of grease at half normal speed with a DC 10 Acrobatics check. Failure means it can't move that round (and must then make a Reflex save or fall), while failure by 5 or more means it falls (see the Acrobatics skill for details). Creatures that do not move on their turn do not need to make this check and are not considered flat-footed.
Difficult Terrain: While there is no official definition, difficult terrain is implicitly defined as ground which hampers your movement (1/2 or sometimes 1/4 speed), but which you don't have to make a acrobatics check to move through.
Answer: No, Feather step does not negate Grease because Grease is a step up from difficult terrain. Amongst other things, it not only hampers movement, but it also requires an acrobatic check to even move at half speed.
What you would need is the freedom of movement spell to negate the effects of Grease.
