The stuff in the last picture looks like FA to me, but if it's the same stuff that is on the hoe it's not FA, and I have no idea what it is. If Glyphosate doesn't kill it, I doubt 2,4-D will.
I would try generic Clipper - the active ingredient is flumioxazin. Or Green Clean Pro. Typically you use Cutrine Plus liquid for 95% of the algae that is out there, but if used too heavily it will kill Koi. I use 1 quart mixed in 2.5 gallons of water and spray around the edge of a 3/4 acre pond that has Koi in it, but I'm only spraying the 3-4 feet closest to the shoreline and the Koi that are in it are fine.
You could also try going here and see if you can ID it.
https://aquaplant.tamu.edu/plant-identification/
