New visual cue, similar to cave dust falling for caveins.
If a mined tile within 5-10 tiles spawns slime, then there is a chance for the mined tile to have slime droplets falling from the ceiling. Not appearing 100% of the time, and not just immediately adjacent tiles because it should be vague enough to still make it very possible to accidentally mine them out even when trying to avoid them, and also so it isn't worth it for mass miners to try and avoid the area.
Basically an early world game mechanic for cautious miners (those crazy folk who never use supports).
I have a feeling that slime spawning tiles aren't a predetermined thing like the cavein tiles, but rather a %chance upon mining out a tile, so that would need changing, but otherwise not much going on.
Normally I'm very pro slime spawning death but I saw some dirty city muck dripping from an overhead railroad and it seemed like a fun mechanic that would be useful for cautious slow miners and be ignored by late game miners or those with fighter support.
Edit: Perhaps the radius (and even slime spawning %chance in the nearby area) could be larger for the slime dungeon entrance?