Why coraline algea doesn't grow on rock?

[WatDatThing]

Ok, my tank is only 5 months old. But! I have coralline algae growing all over the glass on back and the sides. Why are they not growing on the dry rocks? Is it because other algae grows there?
And speaking of algae, I have brown spots slimy to the touch algae growing on the dry rock. However, those are absent on the live rock. Interesting right?

 

[Sk8r]

Because the glass is easier. Murphy, sorry to say, is in charge of tanks. If a fish jumps, it'll never be the one you regret buying. Glass is also fairly chemically neutral, and there may be something about the rock. If you have truly dry and not-cured rock in there, hair algae will love it, but when coralline does go to the rock, it'll likely be the cured rock.

 

[WatDatThing]

The dry rocks were not cured. I thought I read that the rocks only need to be cured when adding to an established system. I'm glad I ask. I will need to move to a new tank soon so I have an opportunity to fix it.

 

[Sk8r]

Limestones CAN release a load of phosphate some months after you thought everything was safe, and it can be pretty bad. Not all limestone has it, but some does, naturally, straight from the ground, just one of those little gifts of nature. If you can set them in to cure and still have enough live rock, great; if not, prepare to run a lot of gfo down the road.