A small amount of bleach might help. There is actually an algae remover available at pool stores.
I do use Algae Fix, that says every 3 days but I tend to forget a few days and before I know it, it is out of control. Also by killing it off then it creates sediment on the bottom that needs to be vacuumed out and I just don't have the strength to do that and the slight bending over to do it kills my back. I have had a pond guy come and drain it and power wash it a few times but that costs me about $150. He doesn't do routine maintenance.
The biggest problem is we had a landscaper company put it in and we didn't know much about Koi ponds so there is NO bottom drain to drain sediment off. If I shut it down that would mean having to move all the rocks and filling the pond area with dirt because if I don't then rain water would collect in it and standing water creates mosquitos. Can't win for losing. :-)