Cyanobacteria. Cut way back on feeding, siphon it out when doing PWCs, add some more flow to the area.
What kind of light do you have over the tank. It could also be from old bulbs that have changed spectrum.
bacteria is brought on from excess nutrients, low water flow and the wrong spectrum lighting can add to the problem also.
Just to go through the litany once again:
1. cut back the lights to 4 hours or less /day
2. cut feedings to every 2-3 days
3. cut amount fed at feedings
4. continue to do 20% weekly PWC
5. vacuum out the cyano
during a PWC
6. test water for PO4
and it tests above .03 start using GFO
, Phosban or similar
7. test for nitrate and get 0 for a reading
8. replace all bulbs (PC
older than 6 months, MH
over a year, etc.)
9. add or move a PH or two to achieve more flow in the affected areas
Chemi-clean by Boyd Enterprises. It will take care of the Cyano
in your tank in just a few days. It is reef safe. It is a last resort. You need to get to the source of the problem or it will come right back.