Macroalgae growth is a little more complicated than simply putting the rocks into the tank.
Coraline algae requires four components: calcium, good lighting, proper ph, and proper alkalinity.
You will more than likely need to supplement calcium or limewater (kalkwasser) into your tank to keep the calcium levels from being depleted. This requires weekly testing though.
However, you can have all the calcium in the world in your tank, but if the ph and alkalinity are not tested and kept optimum, then your coraline algae will not be able to utilize the calcium.
If the ph is too high, then the calcium will turn into calcium carbonate and actually be counterproductive to the macro growth. If your alkalinity is not the right level, then the macro will be unable to draw the calcium from the seawater.
The worst part about having the incorrect alkalinity/ph levels is that the corals and coraline algae will get the ions they need from the carbon dioxide in your tank. This will lead to insufficient CO2 saturation (yes, you DO need some CO2 in your tank) in your water which will significantly raise your ph. When the ph rises too much, you go back to square one of the calcium turning into calcium carbonate.
Hope this helped.