Definite would say cyano
. I hate that stuff. Took me 6 months to rid. I did a lot in that time to try to rid it and I can't say exactly what did it but here is the list.
Weekly water changes instead of bi-weekly.
Switched from phosban to phos extract
Lessened light cycle and changed bulbs
I siphoned out as much cyano
as I could each water change.
Changed flow to increase in cyano
I also never showed any reading of phosphates. Had only 5 ppm
nitrates. So I think it consumes faster than test reads like you said.
Do you have refugium? Direct sunlight? Old bulbs? Low magnesium? Phos reactor? Some questions to ask yourself. I hope this helps. Also nothing in my tank was effected by cyano
. Was just matter of looking bad.