Koro communicating with the algaeic slime mind
The algaeic slime mind was a sentient algae-based entity that caused the Great Soolian Algae Crisis of 2341. The crisis was resolved by Federation Ambassador Koro, a sentient octopus who successfully communicated with the algae at the Starfleet Aquatic Research Center. The Algae Crisis figured prominently in Brad Boimler's 2381 holonovel Crisis Point II: Paradoxus. (LD: "Crisis Point 2: Paradoxus")
It is unclear whether the algaeic slime mind was an actual historical entity or a fictional creation.