HMA is short for Heavy Metal Axe.
Basically this refers to a resin based water filter used to filter out chlorine, metals and other contaminants in the water without removing minerals and salts. Therefore does not change the GH of the water.
They usually consist of a ion-exchange resin which removes metals like copper and lead followed by a fine filter for removing very fine particles (usually around 5 micron in size). Followed by a chlorine.
Typically these filters are used for keeping some fish like Discus which are highly sensitive to the presence of metals and chemicals in the water which attack their nervous system.