I spent a little time yesterday checking monsters in the d20 SDR. Apart from a handful of spelling variations, all the creatures in Monsters & Treasure appear in the d20 SDR. Eventually, I found green slime and yellow mold in the dungeon hazards section of the SDR, rather than the monster lists. I was happy to find them, as I thought I might have to substitute some real-world or mythological monster.
The “snotties” found in Cueva de Villa Luz would make a reasonable real-world substitute for green slime. I believe (correct me if I’m wrong) they are archaea, a biological group that includes organism living in volcanoes and other extreme environments.