seems to just show up overnight in small patches. It does seem to look like a really thick diatom clump at first and then becomes obvious cyano
I havent had it in some time. It just kinda went away once I reduced my feedings quite a bit and cut back on lighting.
If you have already done these, you might try some macro algea like Chaetomorpha to out compete the cyano
Id also stay away from chemicals except for those that simply absorb nutrients. I have read about people really screwing things up with the magic cure products.