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 Can also use Ni(2+), Co(2+) and Fe(3+). Inhibited by BSH. kcat is 42 sec(-1) with GlcNAc-Mal (PubMed:20799687). kcat is 8.24 sec(-1) with GlcNAc-Mal (PubMed:23758290). Deletion reduces bacillithiol levels by 30%. Belongs to the PIGL family.