Sub Grey()
Application.ScreenUpdating = False
Dim Tbl As Table, Cll As Cell, i As Long
For Each Tbl In ActiveDocument.Tables
  For Each Cll In Tbl.Range.Cells
    i = i + 1: If i Mod 500 = 0 Then DoEvents
    With Cll
      If Split(.Range.Text, vbCr)(0) = "-1" Then
        .Shading.BackgroundPatternColor = wdColorGray10
        .Range.Text = ""
      End If
    End With
  Next
Next
Set Cll = Nothing: Set Tbl = Nothing
Application.ScreenUpdating = True
End Sub