An interesting idea and although I am glad you didn't make another midrange attacker like everyone does, I think I am missing the use of the card. Does the polyp multiply itself? the wording is a little hazy.
The Polyp creates a Jellyfish and turns in to a Jellyfish itself. So, essentially, you're getting 12 damage (unupped) for 6
and 6
. Unupped, you're probably looking at using it for fodder for Scarabs, Immolation, etc. Upped, you can fill the field with them, and they can grow at an exponential rate.
I did originally think of having them both be Immortal Jellyfish and just having the Upped version have better stats, but thought I'd see what people thought of the Upped version gaining an ability the Unupped doesn't have.
If it does spawn new creatures it could have a use with SoP but seems like a very slow process.
I did wonder whether it was too slow for the stats I've given them. Perhaps I should buff the Jellyfishes to Firefly type levels?