Boom... there's the issue... po4 and no3 severely out of whack.
If I were you I'd be running gfo in a reactor to get those po4 numbers down. During that time, hold off the water changes to get no3 numbers up a little.
Aim for no3 being 100 times the po4 value... so if your po4 is reading 0.04 then no3 should be about 4... it isn't exact, but close enough to work.
Don't clean the cyano off in the menatime, you want to see it stop growing
Once it's stopped growing then treat with h2o2 to get rid of it once and for all.
Use h2o2 to treat whole rocks removing and putting into 20% h2o2 to 80% tank water for an hour. If no corals attached this is fine. If you've got corals attached, spot treat using a syringe or turkey baster. The cyano will turn white on contact and then bubble away for a few minutes. It'll the back to red... don't worry, it'll break up over the next 24 hours. Don't overdo the h2o2... in your tank size don't dose more than about 5ml per day... it takes 48 hours to breakdown into h2o and o2 entirely.
Should be all good after that
If you're running on a sump, once you're done treating, get some macro and a grow light in there... not only will it help to control po4 and no3, it'll keep those nutrients in balance too by using them up at the correct ratios.