I have been dealing with the same problem in my tank. I finally paid someone to come in and do a full diagnostic on my tanks parameters. I was told a few things will help this.
1. Place your live rock on the glass and add the sand around it. He said the live rock on the sand will cause a compression of the sand resulting in a sand bed that cannot breathe properly.
2. Do a water test on the water you are adding. I thought I was being smart and using the water from the RO system at the grocery store. He said that water could have a phosphate problem then every time I top off I am adding food for the algae.
3. Do more frequent water changes, I was doing one every three weeks of 30%, and he said to go to 10% once a week.
4. Change your filters more often, I was changing once a month in my canister filter, He suggested twice a month.
5. Start running carbon again, which I stopped running
6. Check you water PH, KH, Magnesium, Alk, and calc and let us know what they are.
7. Run your lights for six hours a day max
He also suggested I might have a low oxygen level since a main food for algae is CO2. He told me this; Imagine your tank is a glass of water, now add oxygen, phosphate, magnesium, calcium and salt you that glass of water. There is only room for so much.
I would be curious to know what your Calc and ph are doing, I have been having a tuff time raising mine and I am not sure why. He told me to test my Magnesium, which I was not doing, and I found it is low only at 1050, the test says a normal range is 1500. He says low magnesium will not allow my ph and calc to increase.
Hope this helps because I am not an expert by any means but I have the same problem, maybe we can figure it out together.
One other thing, have you noticed you coroline receading or not growing like it used to?
