Cyano aka Cyanobacteria or red slime. Start with one step they try the other. First there has to be excess nutrients in your system otherwise it would not grow. This also goes for algae. Nutrients: Phosphates (PO4), DOCs (Dissolved Organic Compounds), and nitrates (NO3) are primary nutrient food sources for red and other slime algae. I would test the tank water (although it may not always show high levels of any Phosphates or Nitrates because they have been used up by the algae) and also test your fresh salt water you mix up. Are you using RODI for it?
3 days with out lights will help control the cyano. But as everyone was saying it is most likely from excess nutrients in the water. So you need to look at how they are getting into the tank and reduce that. Could be from excess feedings of livestock, or from poor quality of water used for top off/water changes.
The addition of dry rock now will only complicate issues within your tank right now. With any rock you add to your system you don't know if it will add any excess nutrients and start the cycle over again. You would best off if you could take the rock and in a small tub with a power head and fresh salt water and let it sit for a week and test the levels. If it shows any PO4, ammonia, nitrates, nitrites then think of what you avoided added to your tank. If the PO4's are high then you may want to look at giving the rock an acid bath to remove it. Anything with ammonia, nitrate, nitrites just means the rock wasn't as clean as you though. Easier of the 2 issues to deal with, just let the rock continue to cycle before you add it to your tank. If the levels come back low then go ahead and add them to your tank.
