There are a few quotes from Star Trek that aren't actually ever said (such as "Beam me up, Scotty"). Is there an actual episode where Spock (or even McCoy) says something akin to
It's life, but not as we know it.
Looking through this site which has transcripts of all Star Trek episodes the line does not appear. There are a couple of lines that come close though:
From season 1 episode 26, The Devil In The Dark:
And later in the same episode:
From season 1 episode 29, Operation: Annihilate!:
In "Errand of Mercy" from TOS (S1E26), after the Organians reveal their true nature as beings of pure energy and literally stop both the Federation and Klingon fleets dead in space, Spock says something to the effect of: "Not life as we know it at all".