Do you have minnows and or grass carp in your pond? If not you may want to consider both as the minnows will make short work of the larva, bugs , etc. and the carp may help with the green stuff. Sounds like it may be from the algae. Also the minnows love the plankton (algae) as well.

What other fish do you have? Are you sure about the disolved nitrogen? Someting is feeding that green slim that turns to muck. If there is too much then that would explain over growth of the plant material.