Try this change (added line beginning "ActiveSheet")

Private Sub Worksheet_Activate()
Dim c, x As Range
ActiveSheet.UsedRange.Interior.ColorIndex = xlColorIndexNone
For Each c In ActiveSheet.UsedRange
If Not c = "" Then
Set x = Worksheets(1).UsedRange.Find(c.Value)
If Not x Is Nothing Then
c.Interior.Color = RGB(0, 255, 255)
End If
End If
Next c
End Sub