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 converts slowly into Polluted Oxygen (42.5 g/s) at a mass rate of 1:1. Slime stored in the open, in storage, submersed in liquids or under solids without enough pressure will convert.
Until you have researched Algae Distillers and ways to deal with Polluted Oxygen you should NOT mine towards slime or areas where Pufts live.