I've used rock in freshwater tanks that had lichen/moss I couldn't power-wash off. It would just grow a white fungus like stuff over it then disappear in about a month.
I'm sure their are people that would disagree with this, but I've seen a few posts on RC where people claim base rock is often full of phosphates that leech out over months/years. Supposedly the best way to deal with this is an overnight soak in vinegar. That should also kill/dissolve anything organic living or growing on the rock. It might be worth a shot.
Edited by trunks - April 04 2009 at 6:22pm