cyanobacteria!

Cyano problems are usually caused by aquarium water with excess nutrients like phosphate, nitrates etc. You can try and siphon out what you can to remove the algae in the short term. In the long term you may have to examine how you care for your tank. There are some commercial dry foods also contain excess nutrients and other undesirable things like nitrates, phosphates etc so I strain/rinse my fresh/frozen foods before feeding my tank. You may want to consider feeding your fish less often. Also, changing your photo period, light bulbs, adding more flow, using RO/DI water with a good salt. If your phosphates are high you can add a phosphate remover like Tropic Marin Elimi-Phos also, increasing flow in your tank may help too.

 

Just a point of interest, when cyano is in bloom, you may find that your parameters all look good. I think that is because the cyano quickly eats up all that is available. You still may have to take measures to reduce phos, ect to starve it out. I had a small outbreak but I was feeding 3 times a day and 10 hrs of light. I'm lightly stocked, so by just reducing light and feeding, I was able to overcome it. I still see a small amount on the hob siphon tube. Back them my perams were better than they have ever been.