his second ability should be -x. the colorless control is an extremely limited ability against most decks, and without use of that all he can ever be is an easily destroyable land
I see your point, but I don't want him to be too powerful for the 0 cost. Sometimes he just a colorless lotus petal, but he can be so much more.
maybe you could stretch the planeswalker bounds and make all his abilities take away loyalty counters (at activation). tack a -1 on his first ability and he can only grow on certain turns. he'd still effective as a single mana boost early on, but there'd be a big incentive to play him later when your defenses are set up and you can actually cast enough spells to have him grow
then the minus -x wouldn't be unreasonable at all. it would require finesse to execute properly but still be super useful against non-artifact heavy decks
The problem with making the first ability a -1 is that he would die before you could put other counters on him. I like that he's almost "just" a fragile mana source for 0. It puts him in a unique position to be played as such.