It is carbon dosing. I use NoPox by RedSea for carbon dosing. Carbin dosing is feeding a controlled bacteria bloom. The bloom uses up nitrates and phosphates. If an overdose occurs it causes a bacterial bloom. The bacteria is not the bad part, but the bacteria does use up the oxygen, if it is bad enough it can cause a significant drop in pH. To fix you stop dosing do water change wait to return to normal and start again.
Yes, it is also common to see cyano growth with all carbon dosing.
I can easily dose my mixed reef 125 but my softie tank with my seahorses looks like this within 2 days everytime.