I live in a small town surrounded by country

Of course all our fertilizing was done in the spring, fields still have standing corn and beans. I don't cover my pool, but I don't know where silicates or nitrates would come from, have only used bleach. Having owned a pool for 12 years, this is the first year I noticed this stuff, and is also the first year I added salt. Coincidence? Perhaps the silicates are in the salt?