I think that is waaaay overpowered my friend. Just the simple aspect of "Hides in gravestone at the end of the turn" should make it cost more. It is a zombie that can heal itself no matter what, continues to buff a zombie over and over, and is practically untrickable.
(You know, since it hides in a gravestone in every plant phase. The plant heroe can only kill it with other plants, but you can add some removal like Alien Ooze to prevent it to happen, so this zombie continue fortifing other already fortified zombie).
If I had to create a card exactly as you described it, I would make it legendary and cost 4, like minimum.
But I like the idea. I think it may be more powerful on an already hard to kill zombie like gargantuars rather than on pirates, but it may still work pretty fine.