I had a problem with hair algae, not to the extent in your pictures, but it wouldn't go away. My tank is also older than yours. I used GFO, did big water changes, carbon dosed, and even tried reef flux. While I was succeeding at keeping the algae in check, it just wouldn't go away... and I was also denying my corals nutrients they needed. I was having some growth & color issues.
As I thought about it, I realized that the nutrients that were feeding the algae were not solely bio-available N&P. The algae was competing for other nitrogen compounds that exists for short periods of time and finding the P it needed leaching from the substrate inside the tank.
As there really isn't a way to reduce these available nutrients, I chose another route. I chose to try a relatively new product... Vibrant. I won't claim to know what is in it, but it somehow employs bacteria to control algae directly. I kept up the GFO and carbon dosing, and did a slightly larger water change with each weekly dose of Vibrant. AND... I added a Sea Hare. The algae in my system was gone in 4 weeks and has not returned. The Sea Hare went to a new home, and while I do have a little GFO in the system, I've quit carbon dosing. The Vibrant didn't hurt anything in my SPS dominate tank.
