Please help- how to clean

We just moved into a development and this nasty looking pond RIGHT in my back yard needs some love. It has a sewer pipe that takes water run off into the pond from the left, and there is a grate on the right that connects to a creek- but it appears that the slit on the grate is too high to drain the standing water.

So- what’s the best way to get this water to not collect the muck?