just remember that every so often, people break down tanks because they failed at the hobby, for one reason or another, and you might be purchasing rock loaded with nuisance algae, covered with pest anemones, red bugs, acropora eating flatworms, mantis shrimps, predatory worms, dosed with copper...and the list goes on.
if you are going to purchase rock, i would want to see it in a running tank, and i would also want to see that there are bristle worms and/or pods
scurrying about (this would prove that there was no copper treatment).
as for the algae, you can "cook" the rock. this does not mean to put it in a pot of boiling water. it simply means to put it in a bin with a heater and power head and completely cover it so that no light gets in..and you'd leave it like this for a couple months. this will get rid of most if not all of the algae. i still don't know why they call it "cooking".
as for the predatory creatures, raising or lowering salinity is effective.