Did you add a water conditioner after you refilled the pond? Some of these leave a residue.

By cleaning, I hope you only meant that you drained it to remove debris from the pond, and that you didn’t use any cleaning products (no soap should be used). When the water level was lowered, you might have disturbed a pocket of decaying leaves from pond plants or fish wastes. The decomposing material can leave an organic film of dissolved proteins and bacteria which feed on it. I tended to get this in a tank of catfish I have, and found a surface skimmer that attaches to an aquarium filter that got rid of it by drawing water off the surface: http://www.thatpetplace.com/pet/group/10591/product.web You might be able to attach something like this to your pond’s filter with a PVC adapter.