No the rock wont be harmed, only algea.Remove the rock and drench the algae with 3% hydrogen peroxide outside of water. let the rock sit out of water for 5 minutes making sure the algae stays wet with hydrogen peroxide...add more if needed. Do not do any scrubbing, just make sure the algae is saturated with hydrogen peroxide. After 5 minutes, dip once real quick in rodi, and place back in the tank right away. You should see the algae releasing tons of bubbles. Just leave it alone now and in about 3 days the algae will be 80-100% gone. If there is any leftover, repeat this and it should be taken care of. Dont let the hydrogen peroxide hit corals though, it can stress them.