Mcgyvr is giving good advice - if you are going to soak the rock in peroxide, make sure as he suggests to not do all the rock at once, which will kill all your beneficial bacteria, cause a mini-cycle most likely and wreak havoc for tank inhabitants. Whether you use peroxide or not, some manual removal is key, however, because as the algae die, it will essentially be putting whatever nutrients it has already taken up in its tissues, back into the water. (this is a similar problem with turning off the lights and hoping the algae dies) If your filtration is improving as part of your tank revival - great, it might not be an issue, but consistent manual removal with nutrient export improvements is a good double-pronged approach. Make sure you add a filter sock to your return when manually removing - increase flow and trap all the algae bits in the water. I good hard toothbrush will work, or a steel brush is even better, but take off some coralline as well...