Hoppy,

Why do you think that you see the algae growth on the older/dying leaves? What is triggering the algae to grow in these areas?

I wonder if the actual process of the plant leaves dying is releasing nutrients (ammonia -> nitrogen) that the algae can take advantage of. I can't guess as to what in the actual growing process would inhibit algae growth. So... is plant growth inhibiting algae or is lack of growth promoting algae?