Slime mobs do not currently spawn in Green Swampland biomes as they do in Default swamp biomes as of 1.4.*

Why not edit the vanilla code? (is noob)

@redwaller66 because:

1. Make mod less compatible. Potentially breaks stuff.
2. Cannot be open source because you cannot redistrubute Mojang's code that was edited

I have added entity spawning hooks to forge that can be used for this purpose.

Requires vanilla code changes. The slime entity has the vanilla swamp biome hardcoded within it as a valid spawn biome.

Can this be implemented now since Scott wrote hooks for it?

I left right after my hooks were added. This probably needs to be revisited now.

Slimes were added to GreenSwamp in 93c0a69 (March of '13). Is this still an issue?

Yeah we need to add an even listener for LivingSpawnEvent.CheckSpawn, as the slime spawning code (EntitySlime.getCanSpawnHere) does a test to see if the biome is swampland, and if it is not then it will not let smiles spawn above 40.

Adding a listner for LivingSpawnEvent.CheckSpawn that returns Result.ALLOW when the biome is our green swamp and the same conditions that the normal swamp uses are set should see them spawning in the biome at night.

Fixed in 99731e6.