best way to get old dead rock white again???

[Quagmire]

Vinagar disolves calcium.So if you do it becarefull,you wouldn't want to get up the next morining and find a pile of mush.But maybe it will work to disolve the outside layers with the algae on it.But I think after you would want to put it in fresh water to dilute the vinagar that seeped into the holes in the rock.Plus the vinagar will act as a carbon source for bacteria,will be a good kick start for your cycle,but make sure you do a water change after cycling.It may cause a large bloom and use up all the oxygen.