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 kcat is 10.8 with GlcNAc-Mal. Homohexamer. Trimer of dimers. Belongs to the PIGL family.