Your light surface scums look just like mine (in the beginning). I believe it starts out as a Protozoan Scum, that feeds a green algae growth, and if no rain event or good winds occur...it turns to a bacterial thing (maybe). It all stays very much at the surface throughout its lifecycle. many times a nice breeze will come or some rain and the "stuff" goes away regardless of the phase that it is in. I have difficulty believing that it is heavy nutrients related as my pond is very new and past tests have shown no nitrates/phosphates. My theory is that without any wind/rain action that my pond develops a floating layer of pollens that feed the scum. That's all my inexperienced mind can come up with.

Here is a thread that I replied to a couple years ago that has some good links to my other "scum" threads that may be of some interest to you...

https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=513675

In the above link, you will find a link to a Field Guide for Scums. I'll post it here for convenience...

https://www.townofchapelhill.org/home/showdocument?id=28866

I base my assumptions on this guide. Look at page 15 for what your pond may be doing.

I still do not have a solution aside from praying for rain every few days during the hot and dry portions of the summer. I do not treat my pond with chemicals and let it run it's course.