Yuck - I vote cyano too. Usually, it is fueled by phosphates and the tricky part is that you can test for phosphates and your test will show none to low - the reason being that this algea is using it to grow. Try to identify where they are coming from - food source, frozen and some flake have high phosphates and you can try to test them before you add them to your tank - sand bed, try and take a water sample really close to your sand bed (even stir it up a bit to take the sample - or live rock can build it up over time and you can try removing a rock and soaking it in salt water and swish it around really good and test that water.
Try upping your flow, reducing your light period and reducing/identifying high phosphate foods.
Chemi pure (or clean - I always get those 2 confused but the one in the little white bottle) can also be used but the problem is that it will get rid of the symptom but not the cause and there is a good possibility it will come back
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