Hydrolyzes mannosyl-3-phosphoglycerate (MPG) to form the osmolyte mannosylglycerate (MG). 2-O-(alpha-D-mannosyl)-3-phosphoglycerate + H2O = (2R)-2-O-(alpha-D-mannosyl)-glycerate + phosphate Carbohydrate biosynthesis; 2-(alpha-D-mannosyl)-D-glycerate biosynthesis; 2-(alpha-D-mannosyl)-D-glycerate from GDP-alpha-D-mannose (MPG route): step 2/2. Belongs to the HAD-like hydrolase superfamily. MPGP family.