Even if it is OP i think it can be nerfed by increasing cost. I also think this card is very powerful with spells like mitosis and adrenaline. But i see them as good combos than imbalance.
If a card is OP because of a combo, upping the cost will make the card by itself UP. Instead, we should only nerf the combo (i.e. only spams 2 Tadpoles, as poison).
Also, if the ability is passive:
1) A Tadpole can have both mitosis and his passive spamming.
2) Mutated Tadpoles would spam more mutants, thus filling your field with mutants. Cool.
1 is not a problem, since you still have to pay 4
for 1 more Tadpole, that anyways can be easily spammed.
2 can be a problem, but there are two ways of fixing it. Tadpoles can't be generated by mutation (as fate eggs can't hatch into fate eggs), or Tadpole's passive can't go along with mutation (as far as I know, it's a passive).
Good card, I like it.