yes, need more info here. This is all about nutrient import/export.
Import:
How is it being added? (frozen foods - rinse thoroughly, water processing-RO, bottled conditioners, salt mix used, your hands, anything that enters the water at all. Treat this like your in a biohazard situation, and don't want to cross contaminate.
Test your source water just before you top off your evap, b/c sometimes the source water can obtain nutrients after the RO process from something else you're doing or not doing, like cleaning out your RO water holding container.
Export: Good skimmer production daily, at least one Lb of live rock per gal, also suggest using (2) HOT Magnum Canister filters. 1 with carbon and denitrate, and another with phosguard. Run this in your sump for 24 hours at a time, and you will starve this out in a couple of weeks. Change out the media after a weeks worth of usage, and start again. Increase your water flow, b/c once the algae starts to die it will take oxygen with it as it decomposes. You should also have water flow at the surface of your water to move the top layer of water towards your overflows.
Cyano algae is an indication that there's not enough flow through your reef already, so need to plan on much more here at all levels.
I had this same problem in my 50 gal reef. I had zero nitrates/phosphates when testing, but tons of algae including red (cyano) algae, bubble (slimy) algae, turf, valonia bubble algae, and strange fish losses that I could'nt account for. I thought I had enough flow with a 10x's turnover rate, and 2 internal powerheads on a wavetimer...not!
I now have a turnover rate of 40+, b/c of SPS. Not a single hair of algae. I had to use Magnums for a couple of months on and off to keep my Phosphates and Nitrates close to zero after it was cured. If these numbers grow above .2, the natural process is for algae to consume these nutrients and use them for food. If this goes unchecked, the algae becomes prolific, and your test kits will eventually read zero, b/c the algae is so efficient in converting nutrients. Even your rocks might hold bound up phophates for a period of time. Make sure you use a powerhead to blow off the detritus.
We all want to help, but alas I'm being long winded, and still need more info.