I believe its cyano bacteria. I've had the same problem and it was going crazy.
What i recommend; well what i did:
I did a blackout (depending on what corals you have hardy or not; this may not be for you)
i turned the lights off for 3 days straight, and transferred some corals in a smaller tank.
After that they didn't appear.
But if that doesn't work,
i know for sure that Chemi-Clean (blue box thats fairly cheap on amazon) works.
My cyano breakout was a bit much so i had to dose twice.
Read the instructions for further actions.
Water changes after.
Lastly, you have to find out whats causing the problem or it will keep coming back.
Things to consider:
-Length of the lights are on; the longer obviously the more cyano will show because they grow from the lights; they are photosynthetic.
-Overfeeding: overfeeding is another cause, they get trapped in the sand and the bacteria has so much to eat thus causing this outbreak.
-Flow: Some parts that are heavily contaminated may be lacking flow, the food drops and settles and doesn't flow around the tank onto the skimmer.
-Sand: you should sift your sand, there are great cleaner crews that sift your sand: i got a LARGE pistol shrimp, he moves sand all the time, i have a sand sifting starfish, nassarius snails.
-parameters maybe aren't fit.
-temps?
hope this helps.