The reason why Grey didn't attempt to catch the box was quite simple, he didn't trust the female cultivator! From her earlier actions, he could tell that despite her lovely appearance, she was quite shrewd and callous.
Why would he risk something exploding in his face just because it might be a treasure? After dying twice, he valued his life more than anything. Including cultivation arts, magic treasures and the likes.
But that was not to say that he was happy with the sly trick she had tried to pull on him. If he had indeed attempted to catch the box, he would probably be reducing to stripes of flesh and piles of bones.
After concocting poison several times and claiming to be slightly knowledgeable in the field, he could tell from the blackened water that the potion inside was very potent.
It was so potent that it corroded the immediate waters and corrupted the immediate creatures in the vicinity without the lid even being opened. This scenes made his pupils dilate in shock.
