If I cast grey merchant of asphodel and one of my opponent controls witchbane orb, will this effect apply to them?
Yes, hexproof specifically prevents targeting, if asphodel is a global effect(i.e. Each opponent) the each precludes targeting, so hexproof has no effect
So the wording each doesn't mean it targets each of the instances it specifies
that's what I was told
No it doesn't. For anything to target, spell or ability, it has to state target. Choose , global effects, all those go over hexproof.
114.9a (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=114.9a): Just because an object or player is being affected by a spell or ability doesn't make that object or player a target of that spell or ability. Unless that object or player is identified by the word "target" in the text of that spell or ability, or the rule for that keyword ability, it's not a target.