I leave the lights on 24 hours a day in my fuge.
Depending on the type of macroalgae you're using there's multiple reasons why this is the best method.
1) It helps to combat the natural PH drop during "lights out" in the main tank.
2) If you're using one of the varieties of caulerpa (most common macroalgae) this helps to keep it from "going sexual" and polluting the water with spores.
3) Most every form of macroalgae is photosynthetic. More light equals more growth and greater nutrient/nitrate absorption.
I've read about some who like to set their fuge lights to be exactly opposite of their display tank but I haven't seen any data that suggests it's any better than the 24/7 method.
Personally I use chaetomorpha/brillo pad macroalgae.
I have to get it off of eBay cause none of my LFS
ever seem to have it.
The deal I usually get is about $8 for a sandwich bag full plus $7 shipping.
The seller I buy from is lareefs and he cuts some slack on the shipping for multiple bags.
Up to 3 bags is still the same shipping charge as 1 bag and he always adds some sand and rubble that has always had some good pods
I have to say though that the bags get punctured during shipping and every box I've gotten has been wet... everything's still survived so I really can't complain...
Heres a link to his auctions:
Linky - Chaetomorpha Macro Algae from lareefs