I hope maracyn2 is not copper based.
Cyano while a bacteria is also "plant life". It is in every environment on the earth and was what originally liberated oxygen from the earth's carbon dioxide atmosphere.
So it's kinda hard to avoid.
It also has the ability to get it's nitrogen from the dissolved nitrogen gas in the water column vrs the ammonia/nitrate of the more desirable algaes, corals, corallin algae, clams and so on.
so what can happen is the things are going alone really really good, the initial algae blooms/clouds are long gone and nitrates have finally dropped down. (gee through deep sea bed's bacterial action). then all the sudden there is a little speck over there in the corner. and in a week (or so) that cyano is all over everything.
What may have happened is the cyano is taking the nitrogen from the water and thriving. so the tank is rapidily becoming cyano dominated vrs plant dominated.
So kill the lights, the cyano dies off and returns nitrates to the system.
And the system is reset with the nitrogen needed by the corals, corraling algae, clams and other desirables. and they resume control.
At that point you two data. Lights on (at some duration) -- cyano. light off no cyano.
So find a point in the middle where cyano dies off yet the desirables remain.
but then that requires you turn off your $1,000's of lighting.