hi supamarine your welcome I did search of your previous posts and find you have tank 4ft which is 3 months old but firts 2 months you had no light on it and during that stage you had Nitrate level of 40ppm?
This is reason why cyno has waited until now to appear - it needs both lighht and nutrients
Deprive it of either light or nutrients and it recedes or fails to establish
You water parameters and tank specs will help so please update on current test results for SG, PH, Nitrate, Nitrite, Amonia, and Phospate - others is you test for others
also what equipmemt you have filters, skimmers etc as this may help others to advise you in order thios algae bloom recedes and disapapears quickly and others do not take its place
Steve