What you've found is a [Dungeon.][1] Specifically, a Zombie dungeon. It contains a [Monster Spawner][2] which is able to spawn monsters even if `spawn-monsters` is disabled. The only way to stop a spawner from spawning monsters, is to fully light up the area, or set the server to peaceful.


  [1]: https://minecraft.wiki/w/Dungeon
  [2]: https://minecraft.wiki/w/Monster_Spawner