How To Address My Neighbors!?

Hi everyone! Looking for some inspo or help with this situation before taking action. I don’t want to come off as rude given I’ve never talked to my neighbors before other than brief encounters.

Long story short, my husband & I live in an apartment with a private yard. We’re on the first floor, while we have people above us & people above them. The third story neighbors have a beagle, who they leave outside all the time. We always hear him whining & have seen him sticking his head out of the railing when our dog is out.

We’ve had brief encounters with them when their dog accidentally drops toys into our yard that our dog will find & try to keep. We have always given them back & told them they’re more than welcome to come into our yard to retrieve any fallen toys in the future.

LATELY however.... I would say for about 2 months or so, we have been seeing poop in our yard that we know does not belong to our dog. It’s a lot bigger than our dog’s poop & we also clean up after our dog because he will eat his own poop if not.

We recently took our dog to the vet because he was vomiting all the time & we didn’t know why.

The vet told us it was hook worms due to him eating another dog’s poop. We paid for the treatment, exam & for him to go to the vet in general & we were able to successfully cure the hook worm issue & get rid of them, while also trying to keep our yard clean as well.

It’s been hard to prove, but my husband & I have been suspicious that our 3rd story neighbors are either dropping his poop below or the dog is somehow dropping it. The best conclusion we’ve come to is that they’re not picking up after their dog so he’s dropping it or is causing the poop to fall into our yard, such as the toys he was dropping previously.

We know this to be the case because I was sitting outside Saturday morning with my dog while he used the restroom, when several pieces of poop fell & almost hit my dog in the process.

When I contacted the leasing office about this issue, they told me I wasn’t able to prove that it wasn’t my own dog’s poop, therefore there’s nothing they can do, which I suppose is true.

My question is.. how do I go about handling this situation nicely before I have to get rude? I haven’t spoken to our neighbor about this yet but when we clean up our dog’s poop, we also clean up the poop that’s fallen into our yard because our dog has & will bring it into our home or try to eat it. We want to be able to let him out without worrying about him eating another dog’s poop or having to clean up after a dog that isn’t ours.

Do I write them a letter & leave it at their door or try to talk to them face to face? I’m not good with confrontation so this is why I haven’t talked to them face to face yet. I’m getting to the point where it’s just annoying having to come home & find shit all over our yard & on our patio.

Without being mean, can someone please suggest some ideas??