Genesis microbe
The Genesis microbe was a lifeform that inhabited the  planet but was of alien origin (relative to that planet). 

They first arrived in Spock's tube soon after the artificial formation of the planet but still during the Genesis effect. Originally, microscopic lifeforms natural from Earth starships environments like the torpedo tube, they evolved to macroscopic organisms due to the Genesis Wave.

One of these was violently killed by Kruge. ()
Microorganisms