Catalyzes the hydrolysis of alpha-D-mannosyl-glycerate (MG) to D-glycerate and D-mannose. Can also hydrolyze alpha-D-glucopyranosyl-glycerate (GG)with lower efficiency. (2R)-2-O-(alpha-D-mannosyl)-glycerate + H2O = (R)-glycerate + D-mannose (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + H2O = (R)-glycerate + D-glucose Activity is not dependent on divalent cations, but it is enhanced by Mn(2+). Belongs to the glycosyl hydrolase 63 family.