What you have is New Tank Syndrome. After cycling your rock, your tank will have high concentrations of nitrates. I would think that your new rock will also release some phosphates. Also any food added for fish and other tank critters will decompose eventually producing phosphates and nitrates.
Algae needs nitrates, phosphates, water (it IS an aquarium), and light to grow. A new tank is an algal paradise. To curb the growth of algae and to reduce it, reefers use several methods:
1) reduce nutrients by successive water changes. This works by dilution.
2) using a skimmer. This pretty much just gets large molecules in the water. It really does not reduce nutrients as much as it reduces their rate of increase.
3) carbon dosing. This reduces nutrients. You add a source of carbon to the water like vodka or vinegar. The carbon causes bacteria in the water to reproduce like crazy. The bacterial reproduction takes nitrates and to a lesser extent phosphates out of the water. An over sized skimmer removes the bacteria.
4) just physically remove the algae.
5) add herbivores like snails to eat the algae.
6) grow macro algae like chaeto in a refugium. The algae grows and takes nitrates and phosphates from the water. Periodically, you remove some of the algae.
It usually takes 6 months to a year to get algae under control.
