Sounds like cynobacteria. It sucks.
cyno usually results from too many nutrients and over feeding. It will also spread more quickly under strong light.
Fixes;
1) more water flow... how big is your tank and how much flow do you have? I have a 72 gallow bow with 4 maxi-Jet 1200's and I still get a little bit of cyno pop up here and there.
2) Reduce feeding. Try not to use sinking pellet type food, for me sinking pellet type food was causing a nitrate problem and cyno on my sand bed. Best to use frozen food that is disolved in some tank water and added slowly while the fish eat in doses to minimize how much food is left uneaten and spread through the tank.
3) reduce or stop any supplementation you may be doing. In raising calcium levels are you making sure that your ALK is not dropping? Alk and calcium are in constant balance... high calcium levels will drop your alkalinity/buffering capacity and can cause your PH to fluctuate. In general you get enough calcium from weekly water changes that supplementation is not needed. And if you do,.. you MUST check Alk and make sure you are buffering the water if needed. Also, any other supplements you are adding can add excess nutrients to the water which cyno feeds off of.
4) I don't have a sump/refugium... but i believe you can put in things like cheato that will help control excess nutrients.. someone who has a sump could give more input here.
So besides using chemicals, which should be a last resort cause results will only be temporary, these are things to try.
Also do your best to remove as much of the existing cyno as you can with a turkey baster or something that sucks it out while you are trying to correct the situation. It can and will smother all your corals and other living animals.
