Try Patio Magic, but make sure you get the one for fence panels and decking.
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Give the decking a gentle jetwash just to remove the dirt and the worst affected areas. Mix up the patio magic and apply liberally with a soft bristle broom (I used a soft dustpan brush) and allow to soak in and dry naturally.
Did mine about 18mths ago and it's still algae free. Previously the wooden decking was lethal. Walking out the back door onto the decking invited a slide of up to 15' before becoming unceremoniously dumped in a heap on the grass.