Hi JC and welcome to the PB Forum.
The "weed" is actually a form of algae known as Chara, muskweed or by its well-deserved nickname - skunkweed.
You can read more details about this infamous pest
HERE.
Judging by the density and maturity of the Chara in your photos, it may be beyond a successful chemical treatment this season.
Water-hardness (high), excessive water clarity and nutrients are likely major contributors for its abundance.
