There are many differrent cultivars of cynobacteria. The original blue/green cynobacteria was responsible for converting earths early methane gas athmosphere into oxygen. I am most familiar with the dark burgundy. Cynobacteria are imbedded in coral biomass. When coral is nitrogen starved, cynobacteria die there by giving their nutrients to coral allowing coral to survive and grow.
I have found that sediment cynobacteria flourishes when nutrients in water are low. I can only surmise that cyno is very efficient at capturing major nutrients of N & P. Cynobacteria, through a process called “nitrogen fixation”, converts free nitrogen gas into ammonia. Cynobacteria mats can get phosphate in several ways. The mat changes the pH of the sediment water and consumes inorganic calcium phosphate in substrate. The cyno mat will cover living tissue of coral and dissolve it to assimilate and grow. I used to consider cyno as a bioindicator of phosphate in tank water. Cyno gets phosphate from inorganic calcium phosphate or by dissolving organic biomass, which includes coral.
If competitors of cyno require nutrients in bulk water and cyno gets its nutrients elsewhere, then level the competition against cyno by adding nutrients to bulk water. I feed heavy.
Instead take a step back (as you have) and dig in to find out what's going on.
