There's an app called PlantSnap which allows you to take a picture of a plant and attempts to identify it. I've had mixed success with it (perhaps 50% close IDs), but it's free for about five identifications a day, so it's worth a try. I put your picture into the app (a closer up photo is usually better) and the top suggestion was Chinese Banyan, which I suspect is not right. Further down the list is glossy privet, which it describes as having flowers which are unpleasantly pungent, which may be why it attracts flies - though I didn't see any flowers in your picture. Privet can be difficult to remove, and spreads easily through birds eating the fruit. I've had a few battles trying to get rid of it. A neighbour has it, and it often starts growing at my place.