No plantings yet, it was just filled.
Do you mean this is a new pond? If so, there likely isn't any muck to deal with unless there was a bunch of organics in the basin already. If you have this layer of green on top, and high nutrient water in just filled pond, you may need to look into why the nutrient load is high in the first place.
Any fertilizing done nearby that may have washed in? Any agriculture of farm animals in the area of the source of your runoff? You can mitigate some of that by directing water from those sources someplace else or simply don't fertilize in those areas.
When I needed to fertilize the "grass" (if you could call it that) around my pond I used an organic chicken-meal with a non-soluble source of nitrogen with very low phosphorous. I was very careful to not get it too close to the water. It doesn't take a lot of phosphorous to make a LOT of algae so I didn't take any chances. Without that, my grass could barely hang on in the summer. Soil is pretty poor.
