The rock may appear to be clean, meaning free from much debris, however this rock is mined from the ground. Any phosphates in the runoff water that percolate down will bring that PO4 in contact with the rock, and this is the issue with dry rock that is mined. It can be removed with an acid bath, (the quickest way), with GFO or lanthanum chloride, (the slowest way) or time in the aquarium where it will slowly leach out. Algae growth on the rock only is a sure sign that the rock has PO4 bound in it's outer layers.
With the tank already set up and running, the best thing to do is give it time and keep the GFO fresh. The algae will run it's course but there is no way to know how long it will take, it could be many months before it reaches a stasis with the water in the tank. I bought some of the original Marco in 2007 (it was not so clean) and it has been through 3 acid baths, the first one removed the majority of the PO4 and I still think the acid bath is the easiest and most effective way to remove the PO4.
Give it time Jared, it actually isn't as bad as some tanks I have seen.
With the tank already set up and running, the best thing to do is give it time and keep the GFO fresh. The algae will run it's course but there is no way to know how long it will take, it could be many months before it reaches a stasis with the water in the tank. I bought some of the original Marco in 2007 (it was not so clean) and it has been through 3 acid baths, the first one removed the majority of the PO4 and I still think the acid bath is the easiest and most effective way to remove the PO4.
Give it time Jared, it actually isn't as bad as some tanks I have seen.