Only run one time when worksheet open

Just add a worksheet and make it VeryHidden and test for this worksheet and if found then exit the sub.

A VeryHidden worksheet cannot be Unhidden from the interactive mode and in fact the user will not be aware of its existence. Need to UnHide with VBA code using Worksheets("TestSheet").Visible = xlSheetVisible

Do you realize that with a Modal Userform that the code after the Show command does not run until after the Userform is closed? With a Modeless Userform then the code after the Show command continues to run immediately after showing the userform.

Private Sub Workbook_Open()

    Dim ws As Worksheet

    Dim wsActive As Worksheet

    Dim wsTest As Worksheet

    Dim pwd As String

    ViewOnly.Show

    'With a Modal Userform the following code does not run until Userform closes

    Set wsActive = ActiveSheet  'Need to return to active sheet after adding worksheet

    On Error Resume Next

    Set wsTest = Worksheets("TestSheet")

    On Error GoTo 0

    If Not wsTest Is Nothing Then   'Not nothing then is something so sheet exists

        MsgBox "Code has run once so exit here."    'Delete this line after testing

        Exit Sub

    End If

    pwd = "password"

    Worksheets("Confidential").Visible = False

    'Worksheets("Confidential").Visible = xlSheetVeryHidden  'Optional in lieu of previous line

    For Each ws In ActiveWorkbook.Worksheets

        If ws.Name <> "Confidential" Then

            ws.Unprotect Password:=pwd

            ws.Range("C3:D3").Locked = False

            ws.Range("D5").Font.Color = vbWhite

            ws.Protect Password:=pwd

        End If

    Next ws

    Application.ScreenUpdating = False

    Sheets.Add After:=Sheets(Sheets.Count)

    Set wsTest = ActiveSheet

    wsTest.Name = "TestSheet"   'Can use any name but edit test code above to same name

    wsActive.Activate   'Return to active sheet before hiding added sheet

    wsTest.Visible = xlSheetVeryHidden  'Very Hidden cannot be unhidden from Interactive mode

    ActiveSheet.Range("C3:D3").Select

 End Sub

You could perform a similar thing by inserting a value (True or False or any value you prefer) in an out of the way cell in one of your existing worksheets and test for the existence of that value.