Grease is not difficult terrain.
Difficult Terrain is something that makes you spend 2 squares of movement to move one square.
Grease does not do that so Agile Feet will not help.
Let's take a look at the Agile Feet ability:
As a free action, you can gain increased mobility for 1 round. For the next round, you ignore all difficult terrain and do not take any penalties for moving through it. You can use this ability a number of times per day equal to 3 + your Wisdom modifier.
(Emphasis Mine)
Bolded part states that it allows the character to move through difficult terrain at normal speed. Let's look at the Grease spell:
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.
(Emphasis Mine)
Bolded part is not a concept enforced by difficult terrain, and thusly an effect of its own. In addition, there is no phrase saying "As if it were Difficult Terrain" or something along the lines, further enforcing the factor.
With this in mind, the Cleric's "Agile Feet" ability would not work, meaning he must make the Acrobatics DC (and Reflex Save) the same as everybody else.