Is that algae or on the liner ie stuck to it or is it decaying matter that settled there. If it is algae this is a good thing. This biofilm helps keep the Pond in balance and part of a healthy ecosystem.
I would not stick any type of chemical in there as they end up being counterproductive. If decaying matter you can stir it up and let your filter catch it cleaning the media as need be, if you have a filter on the pond.
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