Does the fern algae look like Bryopsis or more like a feathery caulerpa?
Bryopsis:
If it's bryopsis, then it can be difficult to irradicate. There aren't many things that like to eat it. When you are manually removing it, are you siphoning while you pull? There is the chance you are spreading it more. Something else to keep in mind....your tank is on the young side, and it will go through algae cycles before it reaches maturity. Blast off your live rock with a powerhead or turkey baster to help free up detritus. If you get the length of the algae short, then you could always try a small urchin, but these too, can get too large for a tank this size (along with knocking things down). I had a small collector urchin in my 120 gallon that started out about the size of a quarter. It is now the size of a raquetball.
Lawnmower blennies can be fine, however, most of the time once the algae is gone, then the blenny starves to death. Make sure it is eating prior to introduction, or train it to eat while in quarantine.
Hope this helps.