Catalyzes the reversible phosphorolysis of glucosylglycerate into alpha-D-glucose 1-phosphate (Glc1P) and D-glycerate. May be a regulator of intracellular levels of glucosylglycerate, a compatible solute that primarily protects organisms facing salt stress and very specific nutritional constraints. Cannot catalyze the phosphorolysis of sucrose. (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + phosphate = (R)-glycerate + alpha-D-glucose 1-phosphate Belongs to the glycosyl hydrolase 13 family. Glucosylglycerate phosphorylase subfamily.