When you choose "Select All" in a multi-value parameter, SSRS doesn't treat this as the actual parameter label, it will an array of labels from all the available values.
One way to get your requirement is to compare the number of selected parameters against the number of values in the parameter Dataset; if these match, all must have been selected:
="Value: "
+ IIf(CountRows("MyParameterDataset") = Parameters!Code.Count
, "All"
, Join(Parameters!Code.Label, ","))
If you are hard-coding the available values, i.e. not using a Dataset, you can hard code the count into the expression.