We always have a company close down our pool. We open it in the spring and we have never seen this! I called and emailed the company for the past 2 days with no reply. Where do I even start with this mess? I think they forgot to add some chemical last fall. The water is usually clear when I open.
It looks like the pool was covered, so there won't be heaps of soggy compost lurking in the depths. That's the good news. What you need is some test results (your own, not a pool store, and not from strips), us, and a lot of bleach. Ultimately what you'll be doing is called a SLAM Process You can read that article first, or start easy by looking at pictures. Lots of links in Recovering my old inspirational links
Lots of pictures like this, too. Just do what they did.