Pond green film

Hi, Can someone help.

I have Two Ponds in my back garden that are interlinked with a stream, the pump (Titon 12000) is situated in the Bottom pond and feeds water via pipelines through a Bioforce 9000 filter system, and then from there into the top pond. This water circulates back to the lower pond by the stream. This has been established for at least 5 years with no issues and the pump/filter and ponds are constantly maintained, however this year I have gained a surface green film on the lower pond . When I skim this film off the water is crystal clear below off however is back the next day as shown in the photo,also during the skimming there is a strange smell from this film/scum which is quite pungent, But the top pond is remains unaffected. I have been to my local aquatics shop and purchased "duck and blanket weed" killer liquids, and also have just purchased a Blue liquid additive  which apparently prevent Algae growth but nothing is having any effect, does anyone have any ideas please.Kind Regards.

Mick

                                        Top pond  (9 feet x 7 feet  x 4 feet deep)

                        Lower Pond (12 feet x 9 feet x 3' 6" deep)

I have also carried out a water PH test and that was fine, all of the fish are healthy, and so are the plants within the ponds