1996 Audi A6 Coolant leakage

My car was leaking fluid under pressure from the pressurized extension reservoir cap. I replaced it and it was fine for couple of days. and than steam burst out under the hood; It started leaking again. it wouldn't hold a fluid. I found the leak: it was leaking from a valve looking peace on the coolant house on the rear wall right before house enters to the heater core. Picture is attached. ( I was not using the heater) What is the name of that peace? Will my problem would be solved after I replace that unit. Do I need to replace the thermostat? What controls the coolant flow to the heater?