I may have a similar problem. we fill our pond from a well with 5ppm iron. The memorial day flood in Bastrop put a huge amount of water in and through our pond. The problem started about a week ago in our 1/4 acre pond. It now has red scum floating on top, it almost looks like bronze metalflake paint. The mass gets thicker as the day heats up, then around dusk the mat starts turning green. At night is mostly sinks or dissipates.
Under microscope the stuff looks like gold green balls, not volvox, just round uncilliated cells (as far as I can tell with a cheap microscope).
Our soil is mainly sand with a red clay strata anywhere from surface to 10 feet down. We are in the Bastrop forest fire area.
The fish are fine, lots of bream, minnows, some catfish and goldfish. We have skimmed with a makeshift boom once with minimal effect.
Any Ideas on what this is, other than really really ugly.
