Eggs! This is a female anemone releasing eggs! I turned in the other direction and saw this individual:
Sperm! This is a male anemone releasing sperm!
I checked a few more pools and found quite a few anemones spawning today. It's not that common to see this, so I took a few more photos. Can you spot the female and male in the next image?
[The female is on the left; the male is on the right.]
This was a fortuitous circumstance. I'm not sure exactly what cued the anemones to spawn (recent warm ocean temperatures? a nearshore algal bloom?). But I happened upon them during low tide when they were in isolated pools with still water, so the eggs and sperm were concentrated on their oral discs and not yet suspended and mixed in the turbulent ocean water.
When the tide floods these pools, eggs and sperm have the chance to be fertilized and continue development into the swimming larval stage called a planula. Eventually the planula will attach to a hard substrate and undergo metamorphosis into a tiny anemone.
The eggs of Giant Green Anemones are relatively large for marine invertebrates (see close-up below). Each one might be ~175-220 microns across. (That means that you could fit about 5 eggs in one millimeter.) For comparison, Purple Sea Urchin eggs are 80 microns and California Mussel eggs are 60 microns across.
Although Giant Green Anemones are common inhabitants of tidepools along the West Coast, you don't often encounter them spawning.