Brackets, my friend, brackets.
Change:
searchSQL.Parameters.AddWithValue("@username", txtUserSearch.Text.ToString)
To:
searchSQL.Parameters.AddWithValue("@username", txtUserSearch.Text.ToString())
But in fact you don't need the ToString at all, since a Text property is a string already, so use this instead:
searchSQL.Parameters.AddWithValue("@username", txtUserSearch.Text)