Here's my 2-cents:
A) You may have rushed this tank a bit especially with the anemone in there.
B) Cyano is a very efficient bacteria that's been around since the beginning of time. It's survived ice, fire, drought and everything else up to this day. It will utilize any one of (or more than one at a time) several different means to take up or create food. So you're best to attack it from several angles at one time and then after the initial "attack" change your husbandry routine to keep it under control.
C) You probably want to add more powerheads in there to keep the tank rolling better. My own personal pref is to have a min of 15-20x tank turnover per hour for a FO or FOWLR tank and a min of 20x+ for a beginning reef tank. Some people can get away with less but it's really hard to have a well moving tank with rockwork and such with less than that. In the case of less than 15x use more smaller powerheads rather than fewer larger ones. FLOW is your friend
* - Cyano loves low/slow flow areas
* - Cyano that grows this quickly and this thoroughly is being FED well and also there is a strong probability of a water parameter issue (could be phosphates might be something else)
- Do the suggested black out but find a way to cover the tank so even ambient room light is blocked out. Do allow for proper ventilation so don't wrap & tape the tank with black plastic at least not tightly. I've used cardboard, cloth, and even an old blanket to accomplish this task.
- Just before the blackout do as suggested re:blowing off the rock but also use a net to scoop up any Cyano that is on the sandbed. The more you get out up front the better your blackout period will be.
- Keep the tank dark for at least 3 days (my last treatment like this went 5 days and it was a heavily stocked tank with softies, LPS and SPS with absolutely no ill effects)
- After blackout I always start my lights on a minimal light schedule and work up to a normal light schedule over the course of a week.
- After blackout go through the tank again blowing off the rock and getting any Cyano up off the sand bed with a net and then do your large water change.
- Introduce a good healthy bag of carbon in a high flow area (return from DT in sump for instance) for at least a week.
- Increase the amount and frequency of your water change until the Cyano is gone completely. After that you can tweak your water change technique back some but don't be afraid do water changes more often or of higher amount.
