Required for cell adhesion in non-muscle tissues and in maintenance of muscle attachment. Vital for larval development. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 2 manganese ions per subunit. Interacts with Nop17l (PubMed:17007873). Interacts with uri; uri inhibits flw phosphatase activity (PubMed:18412953). Belongs to the PPP phosphatase family. PP-1 subfamily.