After eliminating the causes of moisture, to remove stains in the milder cases it will be sufficient to pass with a spatula or brush with a sturdy brush.

In those cases where the plaster appears to be loose and floury, it will be necessary to remove it with a hammer and chisel until it reaches the masonry.

Clean the wall well with a damp brush and apply the new plaster, possibly more breathable (lime mortar plaster).