What kind of parameters are you running? Pay special attention to your PO4 level and Nitrates. How often do you feed and what are you feeding? What kind of lighting schedule are you running? How old is the tank? A lot of variables can play into it
Check your phosphate levels and post picture possible and if it's growing faster then get seahare and this take will take of your green hair alage but this is temp solution of the problem. Make sure you take actions before this gets out of your hands.....
Tangs need big tanks, most CUC members won't touch long hair algae and really only maintain it. Best to deal with the root problem which is where are these excess nutrients coming from and type and duration of lighting which can fuel it. Not sure what you mean by preventative water conditioning but additives aren't a good solution.
+1 on the above. Of course you'll want to find the root of the problem. That said, the only member of my clean up crew that has any interest in hair algae is my red legs hermit crab. But she does a great job of keeping it under control. I only ever see a tiny bit of it here or there, and then by the next day she's gotten to it. Also, she's pretty.