Minecraft what kind of item is that?

The green thing inside moves see picture

This is a monster spawner, there monsters spawn, if you dismantle it you get xp and I would light it up and put torches on it so that no monsters spawn

Mobspawner. That doesn't always turn green. Depends on the mob or creature or animal (how you take it) in this case it is a zombie spawner. Don't dismantle it! That is a good exp farm

This is a mobspawner. Do not dismantle! If you take it down, it will be gone forever. Just put a few torches around the spawn to prevent mobs from spawning. You can use it to build useful mob grinders from which you can get a lot of items:

More about the spawning itself:

https://minecraft.fandom.com/de/wiki/Spawner

That spawns the respective creature that is in the (cage) make a torch on the mob spawner and then they no longer spawn

That must be a monster spawner what kind of one I don't know exactly in any case a lot of monsters spawn there if you don't illuminate it or dismantle it. You can watch a tutorial on how to build a mob farm on YouTube. This mob farm brings Xp as well as good items. Just search on YouTube:

Minecraft mob farm