Cyano for sure, almost all tanks have ut at one point or another , a goodsized skimmer helps, same with water changes and regular lighting schedules. It mats together so you can remove it manually and it will help. I've had a million outbreaks at one time or another , and I find in an established tank ITs a good sign of something happening in your system. I recently Had a small patches due to snail birth
using flow from a powerhead does wonders, or if you don't have a whole lot of coral maybe a small blackout if it's out of control. Personally I don't agree with chemical fixes