Replying to my own post here and attaching a better picture. I decided this is not pollen in my case. Most of the major pollen here has passed. The blotches are one piece with interconnections, not a collection of pollen spores. They are white and resemble poorly formed snowflakes. In discussing this with a geochemist at work, he suspects calcium phosphate but admits it's a longshot guess. Given we have surficial source water, this makes some sense. I found an article that seems to support his theory. Thoughts? I'm hoping one of the cool chemistry nerds has a go at this.