I had hair algae growing on the Hornwort tethered to the top of my 29. This was in a tank with GBRs and very clean water (~10 ppm nitrate). The way I got rid of it was to throw out the Hornwort. The remaining surface area in my tank was apparently too dimly-lit, as the hair algae never grew anywhere else. I tethered fresh Hornwort to the top of the tank a month later and the hair algae didn't return.
If you're still cycling, consider draping your tank with a dark towel or blanket for a week or so. The lack of light should eventually kill it.