Hello;
Cyano bacteria is feeding on something on the surface of your rock. I use a bottle brush to go over my rock lightly and remove as much as I can when a breakout occurs. And the breakout usually occurs when a change is made to my tank, usually lights, or load and food. I also use a turkey baster to blow off my rocks --- the large ones seem to collect the most as they have the most snail waste on them and my water flow is not high and turbulent. I use antibiotic IODINE --- I start with 15-drops and reduce to 6-drops the next day for about a week. Use this with caution --- it will kill bacteria (good and bad) --- I have enough filter area to not make a noticeable difference. Iodine is converted fairly quickly in the tank to non toxic forms (Iodide and Iodate) and Carbon removes it quickly. (Lugols solution contains Iodine)
Water flow helps as previously stated, as it keeps the waste from accumulating. I am not in the general consensus with large water changes ---- but many find them highly effective (maybe this is due to a good cleaning at the time of water change). My experiences may be due to the fact I have much more filter area to trap and process waste than tanks with live rock alone.
I have over the years found it most effective to increase the number of snails and hermits with a high load tank and large feedings as the snails and hermits are always on the move cleaning.
The safe bet would be cleaning, water changes, and increased flow.
------------------------------------------------------------------------------------
"Put a drop on a slide and look at it with a microscope, if the stuff is moving everything is okay. If is is not moving, change water!"
Enjoy!
OFM