My best guess.... it would depend on the actual composition of the base rock...

We've got a few hundred pounds of what they called Tufa rock (very lightweight and so pourus water will litterly run through it) never much nusiance algae but rapidly covered with coraline. Other Lace rock we have had wet for over a year hasn't really colored up much, and tends to grow a bit more HA and the like...

Just my guess and what we've seen thus far.