97 cavalier a/c problem

i just replace my compressor,filter and evaporator a year ago.But a few days ago, I saw the pipeline going to the evaporator from the condenser(?)is getting freezed(narrow pipe).but it still cools inside the car.do i need to change my filter again?the mechanic says it's the expansion valve but not sure.any help or comment would be appreciated.I just want to make sure what the problem is before i spend on it.thanks..:)

if it is cooling fine it sounds like condinsation frezzing up nothing to worrie about mybe a touch low on freon