Ah, another feature of NAV & RDLC :)
You'll find that the Expr text appears when an expression reaches a certain "complexity" -- e.g. from just a field;
=Fields!TotalText.Value
to
=(Fields!TotalText.Value)
The fields in the header are usually always Expr as the values are passed using GetData and SetData (and remain that way from NAV 2009 reports). While this limitation was somewhat removed in NAV 2013, you can only put static values in the header -- they won't change across pages which is useless for multiple SO's or Customers for example.
There isn't a way to show it in the properties box, however the quickest way seems to be to select the field, Right Click, "Expression".
I'd suggest giving the textbox a name other than TextBox to make it easier to identify fields (e.g. "SalesHeaderNo");