What was your base material on this? Meaning, did the initial cutting you used for it developed submersed or emersed?..
You know, that's what I used to think as well until I had an outbreak in a brand new tank that had almost no substrate, no build up of any organic matter at all besides what was in the cycled filter. I had dosed with ridiculous amounts of ferts upon set up to get the plants going before the cyano. There was just snails and shrimp inside...Part of the ferts was large amount of KNO3. So what triggered it for me is unknown. In an aquarium setup ramshorn snails absolutely love cyano! They grew massive and eventually every bit of cyano was gone either by the snails or the changing water conditions.
It must live of off something.. And about everything that contains water will develop a biofilm and it doesn't take very long. Not even with clean tapwater only to start with.
I remember this from my early cycling days and my semi transparant drinking bottle on the bike. Having the idea i only put in clean tap water and close it, so why should it it get dirty. Than after days unscrewing the bottle i feel the inside is slippery and covered with a coat of slime.. That was rather a chilly experience discovering i was breeding a rather nice bacterial biofilm in a drinking bottle. Finaly clear to me why i used so little nutribars during my bike trips..
Na just kidding.. Anyway from that day on i cleaned my water bottle daily, never tried to wait what the biofilm would turn into if you don't clean it out, probably green at one point and maybe blue after that.
If iit grows at the water surface of an aqarium to the glass or other surfaces it usualy begins with a rather transparent bacterial slime coat that can be a substrate of all forms of algae and other bacteria populations working symbiotic with it.. I have actualy no clue what cyanobactaria eat for diner, it might be other bacteria poop or other dying bacteria. I realy don't know. I do read that water with a lot of organic bioload is prone to thrive cyano.. The only place i see it appear in one of my setups growing emersed is in my lit and planted sump. On top of the first acrylic baffle that overflows water.. It grows all kinds of algae also cyano.. Sometimes i spray peroxide on it and it dies, but if i do not remove the slimy gue it's be blue again within a week. I guess killing it and leaving it is just creating a new nutritious substrate to regrow..
If i remove it, it takes a bit longer but it always reappears, probably after all it's in a dirt collecting sump filter. And a slime coat helps it to collect dirt..