Overview
Cyanobacteria are photosynthetic algae closely related to bacteria. Cyanobacteria have a few different forms in the marine aquarium; dark blue/green, velvet red, and brown.
It is thought that cyanobactera are among one of the oldest known lifeforms on earth with some fossil records of up to 3.5MYA.
You may have seen on the news or first person that some parts of the ocean looking completely red, or blue green. These cyanobacteria blooms are a major problem for our environment, however they only happen when there is a huge nutrient resource for the cyanobacteria to grow.
It is not known why some aquaria have problems with cyanobacteria and others don't. While those that do often have high nutrient levels, many aquaria have cyanobacteria without elevated nutrients and other aquaria with elevated nutrients don't have problems.
It is generally not desirable to have Cyanobacteria in your aquarium as it is often unsightly and few organims eat it. It may continue to grow if the proper measures aren't taken to remove it. In newly setup aquariums its a common occurence.
There are a number of ways you need to attack the cause of the issue.
Removal Techniques
Reduce nutrient input - feed less. Cyano loves fish poo
Increasing water flow on the particular area that it is growing; It will find it harder grow in high current areas. Good flow is essential to any tank and often the existence of this Cyano is a sign.
Increase skimming; skimmers are often an effective way of removing some of it's nutrient sources. Prevents some of it's nutrient sources from growing due to the removal of Dissolved Organic Compounds (DOCs)
Reduce the amount of time the lights are on.
If you have no light dependant animals (photosynthetic), remove the light source completely until the bacteria appears to have subsided
Large water changes; removes the nutrient source. Preferably combined with:
Siphoning the cyanobacteria out. This is not recommended if you have a deep sand bed (DSB), however you can still syphon it off rocks etc.
Often a combination of a couple of methods is the quickest and most effective way of removing it.