Is your green covering Duck weed?
If so just scoop it out wih a fine net.
If it is algae, then I think your best bet would be to use barley straw
along with a biological filter as suggested by howlis.
Barley straw should be used at about 2 Kg/4000 litre and put in a plastic
net underneath the return from the filter It wil ltake about 6 weeks to
work. Also try to ensure that about 1/3 to 1/2 of the surface of your pond
is covered by waterlilies and/or water hawthorn or other surface covering
plants. It may be too late for these methods to work before the end of
september when the water coolin will have a beneficial effect anyway.
Regards
Tony Bull
www.caterpillarfountain.co.uk