Light hitting while moist is what makes the algae grow. Topping off netpots with hydroton may stop it. Are you top feeding or is the moisture from splashing out of res?
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Water level Needs to be 1 to 2 inches below the net basket…
Your going to kill it with your water level being too high… 
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Definitely need to drop your water level down like mentioned above. It probably wouldn’t hurt to get something to cover your hydroton.
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If it were mine I would lower water level as @peachfuzz has stated 1 to 2 inch below bottom of net basket.
Then grab the top layer of hydroton off and clean it then put it back.
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I totally agree, also wondered if pouring h2o2 mixed with water over the whole thing and then rinsing with water afterwards if that would help.