How big is your pond? Ducks produce a lot of waist/fertilizer and they tend to feed on any bottom they can easily reach. That oily film sounds like some kind of algae bloom, either starting or dieing.
FA is a green, stringy, almost cottony looking algae that will grow on the bottom, then break loose and float to the surface. If your ducks are adding too much fertilizer, and your pond is shallow, you could end up with a pond completely covered in FA.
Unless your pond has substancial size, i'd personally recommend finding them a new home.
.10 surface acre pond, 10.5 foot deep. SW LA. The epitome of a mutt pond. BG, LMB, GSF, RES, BH, Warmouth, Longear Sunfish, Gambusia,Mud Minnows, Crappie, and now shiners!!...I subscribe!!
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