Involved in bacillithiol (BSH) biosynthesis. Catalyzes the second step of the pathway, the deacetylation of N-acetylglucosaminylmalate (GlcNAc-Mal) to glucosamine malate (GlcN-Mal). Has weak activity compared with bshB1. (S)-malyl N-acetyl-alpha-D-glucosaminide + H2O = (S)-malyl alpha-D-glucosaminide + acetate kcat is 0.068 with GlcNAc-Mal. Belongs to the PIGL family.