I'm not sure I buy that any significant amount of phosphate would bind to the surface of a coral based rock. Now, all life has phosphate in it (as I mentioned in the presentation on Sunday), so if you have bits of dead life (sponges, soft corals, etc.) on your dried out rock, yes, you should get that off. I think the best way to get that off is with bleach and perhaps a bit of scrubbing with a brush. Now, even coralline algae is going to have phosphate imbedded in it. The natural reef is actually build by fast growing corals dying and becoming glued together by coralline algae. In some cases therefore the rock is going to be inches thick of coralline algae. Could you remove all of that with hydrochloric acid...sure, but the rock would also be gone!!