There is "Season's greetings" or "Merry Christmas" for Christmas. But is there something for Halloween? "Happy Halloween" just does not sound right to me because of the contrast between "happy" and the "pseudo-horrifying" nature of Halloween. And neither does wishing a "horrifying Halloween" seem appropriate. (I'm neither a native speaker nor located in a country with English as common speech.)
There is always the simple and appropriate (and very scary)
No, there is no greeting or farewell saying for Halloween.
Just because of its superficial cultural similarities with Christmas (you get a bunch of stuff that you like), it is not a literal holiday and the cultural practices surrounding it are not about wishing someone well or warding off evil. It is just a bunch of kids running around getting free candy.
There are some language patterns (like 'trick or treat') but that's about it.
Of course, it is becoming more and more popular to have 'Happy Halloween' on banners and advertisements and such, but people just do not walk down the street or burst into a room joyfully proclaiming 'Happy Halloween!'. It's just not on people's minds to have something like that to say, and there's no longstanding tradition for anything (Halloween being a minor cultural addition to All Saints' Day in the Christian calendar).
Some might object to the negative answer, the recognition of a lexical gap, _plus) the desire to have it filled. But consider analogously (in the Christian cultural area) asking the question "What greeting does one use for Good Friday?". There is none. It's not necessary to have one.
So for Halloween, do not feel obligated to try to use any special greeting. Greeting-wise, there's nothing special you need to learn to say (of course unless you're a kid and knocking on someone's door at 7pm, and then, not really a greeting but a request/threat).
I tend to agree that "happy" isn't right, but don't forget that you're filtering it though eyes indoctrinated into a specific meaning for Halloween.
In some cultures (and under different names), it's about celebrating those loved ones that have died; in others, it's to ward their sprites away. In still other similar customs, it's not related to death at all.
However, in northern latitudes, its origin is most definitely related to the cycle of the year—in this time of year, crops are harvested and food packed away for the winter. The whole concept of the relation of death and rebirth (which even Christians have) is pretty universal.
Here is a link that is a little more inclusive than the Wikipedia article (which still seems a bit one sided and pedantic).
Happy Halloween is the greeting because Halloween generally supposed to be a fun holiday. In fact, it is not a true holiday, and you don't have to acknowledge it.
Greetings like "Seasons Greetings" exist to not mention Christmas specifically, because that is noninclusive of non-Christians.
In the U.S., you say "Merry Christmas" instead of "Happy Christmas" but that is an anomaly I can't explain. All other holidays and occasions are greeted "Happy __".