Is there a season's greeting for Halloween?

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).