any chance of a photo please to help us answer the question

Do you have pine trees near to your garden? This is the time of year that the yellow pollen falls from the trees and can look like foam. When I was working I used to expect at least one complaint a year of 'oil pollution' in water, which turned out to be a film of pollen on the surface.

That's interesting to read Bulbaholic, as we live near a forest with lots of pines. We were really concerned how the tracks through the forest turned yellow at this time of year and thought at first that someone had been dumping some horrible toxic waste on the surface until a friend pointed out it was the pollen from the trees, yellow like sulphur.

One year, Bertie, there was a report of a huge 'oil slick' in the Moray Firth which was coming ashore along the Banffshire coast. This turnedf out to be pol;len from the forests in Sutherland that had been carried on a NW wind.

does it look like builders expanded foam? if yes it is a sporing slime mould. nothing to worry about.