Hi Terry
A common problem depending on what additives are used.
Many years ago I had the same problem.
There are a number of except proprietary cleaner available at caravan shops which will neutralise the algae.
So easy will tell you to use bleach or Milton. I don't incase they damage the plastics and rubber seals.
Once left over night with the professional solution, there are a few available, rinse the tank a number times via its filler and drain point. Do not fish into the cassette.
Going forward use Elsan Diamond pink in the water cist ern and diamond blue in the waste cassette.You will never algae again.