As noted in this question, magnesium carbonate is often used by rock climbers to help mitigate the effect of sweating whilst climbing. While this is not a major issue on artificial walls, the residue left by climbers on natural rock is unsightly and has lead to climbing bans or adverse public perceptions.
The above picture shows a hand hold used by climbers (ca. 20cm wide) that is covered in a white residue from climbers chalk. Other likely components of this residue include salts from sweat, oils/lipids from the climbers skin, and skin particles. The rock feature is protected from the weather by the overhang above which has allowed the residue to build up over many years. Conscientious climbers attempt to brush off as much residue as possible (typically using a soft bristle brush to minimise damage to the rock substrate). Adding water to these residues makes them invisible but they reappear upon drying.
Can anyone suggest method(s) for effectively removing MgCO_3 residues from natural rock surfaces without significantly affecting the underlying rock substrate (typically, oxidised quartzite, granite, sandstone, or limestone)?
Alternatively, can anyone suggest a way to conceal with 'whiteness' of the residue (e.g via some staining process or a specific reaction) to reduce the visual impact?
Any method needs to be effectively non-toxic to prevent significant adverse impacts on the surrounding ecosystems.