This is the most likely green "counterspell."

Although it counters anything an opponent controls targeting something you control, it doesn't do anything to spells or abilities that they use on their own stuff. If that's on purpose, you should probably change the wording to match Sheltering Word to prevent confusion. Otherwise, hexproof isn't appropriate. This is one case where shroud is a better mechanic.

As an alternative, the wording on Vines of Vastwood gives hexproof to your stuff while countering your opponents auras and Giant Growths.

Dude, I did think last night that I would rather use shroud, but Wizards has phased it out and I try to stick by their design standards.
Jmgariepy, I looked at Vines of Vastwood last night, too. I considered a variant of that that said "Target creature or creature spell can't be the target..." etc.