I had some fun surface scums going on this year and talked with the forum here (starts about half way down - 7/10/19)...
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=39072&Number=508352#Post508352I never really found out what it was specifically, but it ran it's course. Most of the time, these things are nothing to be concerned about and quite difficult to diagnosis without a specialist getting involved. I found the following guide that, at least, helped me determine that my scum was not the death hand...
Pond Scum Field Guide link...
https://www.townofchapelhill.org/home/showdocument?id=28866The oil film at my pond seemed to come an go with wind activity and, certainly, rain. Once, the pond got neither for several days and it turned into a pretty interesting floaty scum (see thread link above), but was short lived and all is good now. In fact, the surface conditions at my pond have been very nice for the last several weeks/months. I can tell you that the rag weed and mares tail pollen were off the charts this year around my place and it makes me wonder if floating pollen can fuel surface biological activity???
At any rate, welcome to the forums from another Missourian! Sorry I can't answer your questions directly, but, hopefully, I have given you some ammunition.
Is there still this film on your pond now?