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:
SPOCK: Within range of our sensors, there is no life, other than the accountable human residents of this colony beneath the surface. At least, no life as we know it.
And later in the same episode:
SPOCK: Life as we know it is universally based on some combination of carbon compounds, but what if life exists based on another element? For instance, silicon.
From season 1 episode 29, Operation: Annihilate!:
SPOCK: It is not life as we know or understand it. Yet it is obviously alive, it exists.
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".
It doesn't figure because the phrase didn't come from Star Trek! It's actually from the contemporaneous movie, "Thunderbirds Are Go!"