Hmmm....sounds like either an algae or an oil residue of some kind - decaying plant material, amongst other things, can produce this.

Your pond is stagnant, can you obtain a small solar panel as advertised on the internet for about £16.00, make a small filter with a small domestic sieve, put in some charcoal for the water to be pumped into and you should with the circulation and the filter take out the algae etc. before it germinates.

docs right you could have a little fountain to and get that lovley sound . from the picture the pond looks basicly very healthy to me .a good way to filter is to place your pump in a lilly basket thats bigger than the pump and fill with 1" ish pebbles covering the pump . you can still have the fountain sticking out to then . it does work realy well . you could even put in some small bits of barbi charcoal without an excelerent on it ofcourse and put the pebbles over that . take the sponge filter out though as it will clog up quickly .