Answer: I think the soil is compacted and therefore unable to accept the water causing it to run off, which is causing the algae or mold growth. You should be able to aerate the lawn to improve the drainage, which should slow or stop the algae growth. Rent a core aerator which removes plugs from the lawn. Leave the plugs in place - they will dissolve with rain or water from the hose. Spread a thin layer of compost or sand over the area after aerating. This will work its way into the holes left by the aerator and give water a place to drain. I think you'll see a big improvement once you've aerated the slope.
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