Extra people have moved in, what should I do?

[Orion Walker]

Hi Folks,

I have a 4plex and I rented one of the small upstairs apartments to a single person. I recently learned that a girlfriend and toddler have been living there too, or at least there quite a bit at all hours. I really don't want to lose my downstairs tenants who pay more for a bigger/nicer space. It is an old building, so sound between floors carries easily. Month to month rental agreement states that anyone that is there more that 10 days in a 6 month period needs to have written prior approval, or breach of agreement. 

So, downstairs folks haven't mentioned leaving yet, but have noticed increased noise and I get the impression they aren't super happy. Upstairs tenant hasn't checked in about additional people. Haven't spoken to him about situation yet.

So, any thoughts on how I should proceed? Experienced landlords out there, what would you do?

Thanks?