Slime is a toxic biomass that is found in the Swamp biome and Ruins. The tiles can be mined, and can leach germs onto adjacent tiles. Mined balls of slime are also generated by Pufts consuming Polluted Oxygen.
If Slime is not under a pressure of 1.8kg/tile it decays slowly into Polluted Oxygen. Slime stored in the open or storage, or in liquids or under solids without enough pressure will emit toxic Polluted Oxygen.
Until you have researched Algae Distillers and ways to deal with Polluted Oxygen you should NOT mine towards slime or areas where Pufts live.