Fixing green pond water is pretty easy once you understand the role bacteria plays in processing nutrients.
Here's some helpful links:
DIY filters- https://youtube.com/playlist?list=PLJ8Cv9jCFTGTGzqhdCKcWvNxjOyXH6DTy
Helpful video for dealing with string algae - https://youtu.be/xn-V5D045QQ
Pond aerator (amazon link)- https://amzn.to/3P9pJU3.
Solar aerator (amazon link)- https://amzn.to/3nWcvxZ
Pond flocculent (amazon link)- https://amzn.to/3RyMIcH
If you make a purchase i may receive a commission at no extra cost to you.
Diatomix- https://diatomix.com.au/?v=6cc98ba2045f. Use coupon code ozponds for free shipping.
If you don't live in Australia you could try nualgai- https://amzn.to/3gTrt4f (amazon link). It also promotes the growth of diatoms to outcompete problem algae.