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Hypo might remove it but will leave a highly visible difference . I would use a biocide algae remover it won"t work straight away but will workA WFP will spill hypo on all the windowsills plus run down the walls it will be very hard to control where it goes
I would also advise that the guttering and/or downpipe requires clearing out .This would prevent this area getting soiled again in the future Ed