Welcome to the forum, and you definitely found the right place!
I am betting it is the remnants of the break down of rotting plants on the bottom of your pond. As the plants rot and break down, a bi product is that "oily" film you speak of... Can you take a photo of this film? It may help someone ID for certain for you and help give you accurate advice.
Bill Cody the pond doctor may have another opinion on this, it seems to be right up his alley of expertise. Then again, what isn't up his alley of expertise when it comes to ponds?
