TR,
You exchange hellos with your neighbor, so it would perfectly fine for you to go over knock on their door and asked, nicely to not let the dog run free (alone) because he's going to the bathroom in your yard. If they don't kindly to that then you need to call animal control and they will have another talk with your neighbor.
I am a dog lover and have had dogs all my life. I walk my dogs and go to dog parks on a regular basis. I always take my clean up bags with me and the parks I go to have clean up material as well.
People that love their animals and children take good care of them. Sometimes it's kind of messy and not always fun. However, the rewards are worth it.
Blessings....