As far as I know just about all "modern masons" probably use muriatic
(hydrochloric) acid to clean their mortar joints. That is one of the great
things about using lime putty in mortar instead of Portland cement. If you
accidentally slip with your trowel all you will need to do is simply wait for
the lime mortar to dry, then simply remove the mortar with a stiff natural or
nylon bristle brush. No need for harsh chemicals such as hydrochloric acid.
Just be sure you never ever use a wire brush of any type on masonry.

Savannah Steve