I am wondering if this problem may be a new one with Red Sea salt. I used Red Sea Coral Pro for several years when I had my old tank (2009, 2010), with no algae issues at all. Granted, it was a well-established tank at that point. I just set up a new tank in November and started with a fresh batch of Red Sea salt. I have been battling hair algae for months. I use RO/DI, GFO, Carbon, and an oversized skimmer, basically all the same equipment that I had on my old tank. Parameters are all stable. Based on this thread, I am switching salts. It's worth a try at least to see if this is what is causing the algae outbreak.
Could be a new problem. I heard it was good salt so that is why I switched to Red Sea. Sounds like we are having the same problem!