Ab as in Red Sea AB+? What/how often do you feed the fish? These types of issues are difficult to diagnose on a forum so you are in the best position to judge. If it is low nutrients what can happen is that you need competition from other bacteria and algae. Cyano is a very versatile bacteria on top of being noxious so there is almost nothing that will touch it once it has grown out that much. The best course of action is to correct the cause, if it is lack of nutrients you can increase feeding/change to higher phosphate content food such as pellets or algae based frozen food. Formula two has 10x the phosphate of mysis shrimp. You can also dose sodium phosphate directly but nitrate and phosphate are more of a barometer than a cure.
Also make sure you are not overlighting, to strong (the lights in this hobby have gotten super powerful) or to long of photo periods (corals only need 6 hours of light so ramp up to your required lighting but back way off of that for the rest of the day). And finally make sure your major parameters are locked down, Salinity, Alk, Cal, Mag. Coralline is a good barometer for whether your lighting and chemistry is locked in.
Once you have figured that out you should remove as much as you can with a siphon and turkey baster. And then you can add to the clean up crew, fighting conches are the most effective algae eaters and they hang out in the sand but I'd only keep one. Scarlet hermits are great for sand as well as they are one of the best scavengers. And for the rock I'd add a few Astrea snails. The CuC isn't going to solve the problem for you but they are going to help make it better every time you do a maintenance. At first it may be every few days you need to go in their with a turkey baster but eventually things will balance out. This type of thing is very common at the 1 year mark.
