What is the best duckweed or watermeal treatment?
It depends. If you have water flow thru, or other plants that you want to keep, and don't mind spraying a few times, Clipper.
If you don't have water going in and out of the pond, no other plants in the pond that you want to keep, 90 ppb Fluridone. Treat once and maybe bump it after 20 days to ensure the 90 ppb stays there and you're done.
It's growing because of little to no wave action, no weeds/algae utilizing the nutrients and something brought the seeds to the pond, or viable adult plants. It's the worlds smallest flowering plant.