Perhaps the grows on the rocks and spreads out onto the water until it breaks off? But there seems a lot more moss in the water than on the rocks. (Perhaps someone came by yesterday and pulled moss off the rocks and placed it in the water?)
I contacted a byrologist (or whatever the proper term is) and he told me that the moss is probably Pohlia wahlenbergii and it does not naturally form floating islands. He suggests that someone picking it off the rocks is the more likely explanation.
I looked at adjacent pools and found none with floating moss, but also none with this kind of moss at all.
The water in this creek is odd, I presume it has a high sulfur content (after all the falls is called tangerine) don’t know if that affects things.
Map of the area, the text is too small to read here, click on the map to see a legible version. The location of the pool is the blue circled cross just to the west of the “Cold Spring West Fork” label. N34.4666052° W119.6584374° elevation ~400meters.
I’ve never seen moss do this before (if indeed it has) and I’m intrigued.