if it is bryopsis hair algae, alot of guys have had luck with raising their magnesium levels >1400ppm, they say it seems to kill it off, at least enough to make it pallatable for mithrax crabs....at least that's how my friend has had best luck with this method. The magnesium alone didn't seem to do the trick for him.
Phosphates could be feeding into the problem a little bit, and I would measure your phosphates with a good test kit, you want to keep levels lower than .25ppm, in fact maybe lower, natural sea water is somewhere around .055ppm. Of course the catch 22 is that a GOOD phosphate test kit, will cost about 30 dollar anyways! Many of the cheap ones don't measure down far enough to determine levels we strive for.
I had it too once during my first aquarium, and I didn't try to solve it at all, and one day it had finally just run it's course and it stopped. Took about a year. Alot of that hair algae is utilizing nutrients localized in the porous facets of the rock. So attempting to absorb nutrients from the water is probably a good shot at the problem, but may not solve it, although it's generally good to employ some form of treatment for phosphates, ie(refugium, GFO's) .
Furthermore, I've observed that batch doses of kalkwasser may cause localized algae masses to form on substrates. As you probably know kalkwasser chleats phosphates and causes them to fall out of the water column much like sediment. Once deposited it may reabsorb or be utilized by algae or other biofilms.
I guess this is somewhat anecdotal information based upon my understanding of the physical nature of these things, combined with my experiences using them. Ask Randy for the technical, chemistry jargon on the "actual" behavior of these agents.
Good luck, I'm battling a dinoflangellate/cyano myself. We shave prevail!
ltz,
andy
btw: IMO ROWAphos is better, as it is indicated to absorb some silicates, which of course is a building block for diatom algae, but you get alot less mileage out of it, especially for the price! But for general regular use I use PURA phoslock, its cheap, it goes a long way, and it seems to work, my tank measure's undetectable on a LeMont test kit.