Try to get a photo of it Bus. Is it slimy like algae or more granular? One year (2006) we had our Incense Cedar trees pollenate like crazy. It covered almost the entire pond with a light green granular pollen. Looked like heck but is wasn't a big deal. It has never been an issue here since 2006 (we've been in a draught since then, I don't know whether or not that has anything to do with anything). Anyhoo, try to get a photo of it.

JHAP
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"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)