Yep, just a case where the opportunists have taken over, like a pond that has been dosed with salt, or some other catalist causing the balance of power within the pond to change (algae species). Or, the pond may be too low in calcium. I would do a 40% water change, then add some calcium - a small amount of calcium carbonate, or lime, and within a few weeks the pond will likely return to a greener more familiar look.
Best Wishes,
Brady Brandwood
