If it is an old/second hand piece of liverock simply replace it would be most cost effective. If it is a new piece of liverock which has never had algae, check the TDS of your RO/DI unit.
Also, should a hobbyist or LFS have a sea hare for rent, you could use it for a day or two as they are historically known to eat hair algae.
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I'm out. 
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