Hmm. if they are already in a house and you said nothing to begin with i think now its kind of questionable you telling them that you can see what temperature they are using. I only ask my guests in a separate house to not put it on ON or FAN position, the rest i really would not be able to control . I tell them that i can not stress it enough how important it is to not have it on ON position. Also, talking about common sense again, i ask them to not open windows at all if AC is on. Thats in separate unit.
Just to give you an example how they listen to me:
In my house where we live and i rent out 3 rooms, my monthly budget for electricity is 167$.The house is 2100 sq.feet
In a separate house with brand new AC unit, 1600 sq.feet, the same amount of rooms, the same amount of people, monthly budget payment is 302$.
In my house i watch very carefully over my guests and do not let them alter my AC. I had guests who put termostate on 67F, wore long sleeves and covered themselves with sick comforters at night. Some opened windows "to air the room" with AC blasting.
I am on their case every single time it happens. With separate unit like you have its really impossible to control that much and my numbers show it.
Its definitely ok to say something about conserving energy but BEFORE they move in, or even put it in your rules, but not after the fact tell them that you are watching them through a device while you are not on a property