Another thing I find that causes cyno is high organic levels in a tank (food) which then creates high levels of
nitrate etc which will feed this type of bacteria. Also low water flow will allow this stuff to grow. Oh and sometimes lighting, if the bulbs are old they could shift in spectrum. I think if you over feed the tank cut back to maybe once a day and increase your flow in the tank. Some people think they have a lot of flow in there tank but really most of the time it is no where near enough leaving dead spot in the tank. As a last resort I would use the meds they sell to eradicate this cyno but like Rob said it will come back if you do not take care of the cause.
Take care
