It was not automatically active, and could be manually turned on or off.
If I was to try saying it, it'd be something like the suckers on an octopus tentacle.
Apparently, that Hydra seems to activate the magic stone the moment it sees magic; any magic that flies toward it would then be rendered useless.
Those are wonderful reflexes.
But, that would be obvious, of course.
For wild animals, their reflexes and vision are overwhelmingly better than humans are.