Involved in bacillithiol (BSH) biosynthesis. Catalyzes the second step of the pathway, the deacetylation of N-acetylglucosaminylmalate (GlcNAc-Mal) to glucosamine malate (GlcN-Mal). (S)-malyl N-acetyl-alpha-D-glucosaminide + H2O = (S)-malyl alpha-D-glucosaminide + acetate Mutant shows a 2-fold reduction in bacillithiol levels. BshB1/bshB2 double mutant does not produce bacillithiol. Belongs to the PIGL family.