I wouldn't be too concerned about it. My best educated guess, without photos from your pond, is that it is just a dying algae that happens this time of year.
I unsuccessfully tried searching for some photos from my ponds that probably show what you describe. In general, it is a brown slick that gets blown around the pond by the wind. It usually starts with a green slick, sometimes with some red included, that slowly turns brown as it dies. It kind of like in a tie-dyed pattern of shirts from 1969 Woodstock and Altamont.
One of my ponds currently has the issue. We don't swim in these ponds, and we don't let the dogs in these ponds, just in case it is something other than what we expect it to be.
One easy way to get it to sink is to use a water pump. I have a 2-inch semi-trash pump on a trailer. Put the intake in pond, and spray the floating slick. It quickly sinks, and the pond looks normal. It is probably just cosmetic, but the pond will look a lot better. The fish don't seem to care.
Maybe Bill Cody can tell us exactly what it is.
Regards,
Ken
