Plantonic algae bloom most likely. Post photos will help us direct you. Per previous posts, treating algae blooms can be tricky. For folks with nutrient level issues it's helpful to establish good rooted vegetation to help utilize available nutrients which helps manage filamentous algae and planktonic algae blooms. DIY aeration systems for a pond your size won't break the bank and help prevent kill events - strongly consider installing one ASAP.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

[Linked Image from i1261.photobucket.com]