Hi Chloe, is it algae or could it be blanket weed? To solve the algae problem in our little pond I installed a Ultra Violet Light that thickens up the algae which in turn enables the filters to catch most of it and so clear the pond water, on the other hand if it’s Blanket Weed (the stringy stuff) that sticks to the sides of the pond wall then there is a brush like a witches broom that you have to laboriously scrape and turn to scoop it out.
Hope this helps.
Roy
