It's a type of hair algae and I have it in my mixed reef too now. What lights are you running? I have cut back on feeding and dialed down the white and reds on my Radions. It seems to be not getting worse for sure, and where I scrubbed the rock with a toothbrush is looking good. Still have more to go, but I really don't want to pick all my rock apart, as it took me forever to scape it. I did notice, and maybe you will too, that is only growing in the higher light areas, which tipped me off that is was the lights and not a flow issue. What are your trates and phosphates testing at?
Good luck