Regular rock with live rock?

Hi all, I've recently started a 29-gal reef tank, well it will EVENTUALLY be a reef tank currently it is just live sand, live rock, and this awesome rock I found in my backyard. The rock is natural (meaning not some sort of cement or concrete thing) and is full of holes and crevices. I had it in a freshwater tank previously and now have it curing with the liverock. A friend of mine was at his LFS and mentioned this rock, they told him I should not have this rock in the tank as it could hurt the reef setup. I'm not sure what that means.... and I figure this rock would eventually have all the same algae and tiny creatures on it that the live rock has, so how would that hurt my tank? Any clues anyone?

it's lace rock, same type most people use in an african cichlid tank. i used to have it in my malawi tank. one thing i've been considering is the possibility of heavy metals leaching into the water, anyone know if lace rock (don't know if i'm spelling that right or not...) has a lot of heavy metals in it?