Hummm very interesting card, but It's very difficoult to do something like this without being OP
Lets see this: How about giving 0 starting hp, so you'll need malignant cells there before playing the card. Then it grows normally +1/+1 without cost (that's the very nice thing of the card) but I would add: "when no more malignant cells can be added to the field, they kill the spider instead". So you'll always have a limit of growing or you'll have to find a way to keep clear the field.
