Simple fix. In my 55 gallon reef I have almost double the fish you have and feed twice a week.
This will take some time though. Increase water changes, siphon out the cyano
with some airline tubing during your water changes.
Cut back on your feeding, I would get rid of the flakes all together and stick to frozen which is lower in phosphates. I would do about half a cube for those fish twice a week.
It will take some time to lower the parameters down to where your issue will be resolved, so don't give up when this isn't gone in a week. Give it a month of care and you should be in good shape.