It is cyanobacteria. I am fighting some now myself as are a number of other people on the forum. Typically it starts with high light, and high nutrient load of some sort, but as you read other accounts you will find that is not the whole story. There are two main immediate treatments and neither is ideal, you also have to try and find the underlying cause and that isn't that easy.
For the immediate you can either black out the tank for a few days, I used 5 days the last time. Or you can use a product like Blue Green Stain Remover, which rumor has it is an antibiotic, which some people feel we shouldn't use in an aquarium. Either route will get rid of the bacteria in the short term. I have used both but I prefer the blackout, I fear the Blue Green Stain remover might affect the bacteria we harbor in the filter and I don't like throwing antibiotics around. Some people also use Hydrogen Peroxide, hydrogen peroxide did not work well for me.
Once removed if you don't fix the cause, it will be back again. Lighting is a big deal, if you have too much light in the tank it can start again. Your lights should be on a timer anyways so try reducing the time the lights are on, I have gone from 13 hours once to closer to 9 hours now. The other issue is the nutrient load, reduce your feedings (keeping the fish feed) and do more water changes can help with the issue. That said it can have other unknown causes, in my case I am looking at changing the substrate because that is really the only difference between two of my tanks, one with cyanobacteria one without, is the substrate. Some people also mention that nerite snails help with it but that hasn't been my experience.
Note when you clean the tank try to remove as much as possible. Also the bacteria is very mobile, it can move to a better location if need be.
