This looks like Filamentous Algae, but let's see if someone will confirm that.
If it's gathered around the perimeter, a garden rake can remove quite a bit of it, although it's labor intensive. In a current pond I'm helping with, the FA just seems to regenerate quickly. The pond I'm talking about had it's dam cut last summer, and refilled in the later summer of 2021, so we're thinking the FA is just coming from all the leaves that were accumulated on the pond basin, but we really don't know.
You can also use Cutrine or Copper Sulfate on these mats of FA. If you put Copper Sulfate on them, I've found that it just sinks the mat from where it was floating. While this isn't a huge problem, you may have fish spawning under those mats, and dropping the mats on the spawning bed, along with the presence of Copper Sulfate, may prevent that spawning nest from producing.
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