It is an indicator of hair algae and I've been talking with a fellow reefer whos been in it for 20+ years.
GHA grows being having bad/too much/incorrect lighting, nutrients, phospates, etc.
I've had GHA outbreaks in my frag tank and main tank although the stuff in my frag tank is about gone. I was adding phosphates through my kalk drip using pickling lime -- and lagging behind on my water changes over the course of about 3 months on a bad shift.
My main tank has some problem spots of the hair algae. Not "everywhere" but not necessarily a little either.
My readings:
Frag tank --
GHA almost completley gone
alk 4 -5 dKH
Test Kit - API
Main tank --
GHA covering 1/3 or less of my live rock that is in directlight.
Alk 5-6 dKh
Test kit - API again.
Now my reason for wanting to know people's alk is while having a lengthy conversation I told him of my problem and when he heard about my alk levels he brought a new idea up (to me anyways).
While he said the phosphates and everything else obviously need to be taken care of, keeping alk 9+ dKh will affect the GHA in a negative way. I can't remember how he worded it but it's almost like the GHA can't handle those levels and will start to die off.
I haven't tested my levels in those bad 3 months but working on getting all levels in check, added my GFO/Carbon reactors and have been good on my water changes and been pulling at it -- hence my Frag Tank about 100% free and my main tank showing positive signs but alk of 9+ sounds like another weapon in the fight (which ideally I want about 10-11 anyways).
Just some interesting thoughts I thought may or may not be useful out there. If anyone has more comments on this I'd love to hear.