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Okay today I was at my house and entered a flat land map and I made sure that its on peaceful so I joined but then slimes all over the place. So I left and joined but still the, slimes were everywhere and I started getting furious and they annoyed so much that I shouted. This is an actual bug, I saved and unsaved but still SLIMES.
I hope this glitch will be fixed.

I don't believe its actually a bug. But I can't remember for sure. It did get changed/fixed eventually in PC version so I'm sure the same will happen on MCXBLA, you'll just need to be patient. For the time being, there is no way to get rid of slimes spawning on peaceful.

Slimes don't actually spawn on peaceful, however if they spawn on easy or higher and then you save and quit, only to later load the game up on peaceful the slimes for whatever reason will not despawn.

Slimes will spawn only on Layers 16 or below on Easy, Normal or Hard; but will not despawn if you save from one of those modes and then subsequently re-enter the game in Peaceful mode. I repeat, the slimes will not despawn. This is apparently the way Mojang has intended it to be on the PC version and is the same on the XBox version. Since everything on Superflat is below Layer 16, the slimes spawn out in the open and in great numbers on those maps. Since Superflat worlds are really there for just doing creative builds, the problem is best avoided by not trying to "play" them in Easy, Normal or Hard modes; or if you do, then by exiting those modes without saving. I haven't tested this myself; but I do believe they will despawn if you get more than 144 blocks away from them. So, go into Peaceful mode and then fly over to the other corner of the map and they should be gone when you get back. Barring that, you will have to kill them all while in Peaceful mode. I've found that is easiest done from the air with a bow.

Quote from UpUp_Away95

Slimes will spawn only on Layers 16 or below on Easy, Normal or Hard; but will not despawn if you save from one of those modes and then subsequently re-enter the game in Peaceful mode. I repeat, the slimes will not despawn. This is apparently the way Mojang has intended it to be on the PC version and is the same on the XBox version. Since everything on Superflat is below Layer 16, the slimes spawn out in the open and in great numbers on those maps. Since Superflat worlds are really there for just doing creative builds, the problem is best avoided by not trying to "play" them in Easy, Normal or Hard modes; or if you do, then by exiting those modes without saving. I haven't tested this myself; but I do believe they will despawn if you get more than 144 blocks away from them. So, go into Peaceful mode and then fly over to the other corner of the map and they should be gone when you get back. Barring that, you will have to kill them all while in Peaceful mode. I've found that is easiest done from the air with a bow.

Im in the flatlands and also I saved at Esay and switched to Peaceful. This is what im talking about but they appear but also no monsters spawn.....