Negative regulator of sigma-B activity. Dephosphorylates RsbS. Plays a role both in maintaining low sigma-B activity during growth and in reestablishing prestress sigma-B activity after induction. Could have a negative feedback role by indirectly communicating sigma-B protein levels. H2O + O-phospho-L-serine = L-serine + phosphate H2O + O-phospho-D-serine = D-serine + phosphate