Catalyzes the reversible phosphorolysis of glucosylglycerate into alpha-D-glucose 1-phosphate (Glc1P) and D-glycerate (also called (R)-glycerate) (PubMed:28754708, PubMed:29684280). May be a regulator of intracellular levels of glucosylglycerate, a compatible solute that primarily protects organisms facing salt stress and very specific nutritional constraints (PubMed:28754708). Cannot catalyze the phosphorolysis of sucrose (PubMed:28754708). Does not act on other sugars such as alpha-D-galactose 1-phosphate, alpha-D-mannose 1-phosphate or beta-D-glucose 1-phosphate; in vitro D-erythronate can substitute for D-glycerate with a much lower efficiency (PubMed:29684280). (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + phosphate = (R)-glycerate + alpha-D-glucose 1-phosphate kcat is 2.2 sec(-1) for phosphorolysis of 2-O-(alpha-D-glucopyranosyl)-D-glycerate, kcat is 325 sec(-1) for glyceration of alpha-D-glucose 1-phosphate. No visible phenotype when grown on glucose. Belongs to the glycosyl hydrolase 13 family. Glucosylglycerate phosphorylase subfamily.