When salt stays in contact with ice, it drops the water's freezing point, which allows the ice to melt at a faster rate and prevents the water from refreezing. Salt is frequently used during winter to reduce and prevent ice formation on walkways and roads.
If a block of ice is already frozen, adding salt on top of it isn't likely to melt it unless the temperature is already around the freezing point. The normal freezing level of water is 32 degrees Fahrenheit (0 degrees Celsius). However, if there is water present on top of the ice, energy from the water can be transferred to the ice, melting it slowly. Furthermore, the salt water mixture will not refreeze due to having a lower freezing point.