My guess is a scary mask. Arlo might spook the cat, but I’m pretty sure he wouldn’t feed him (or kids, for that matter) something to provoke that reaction.
He could have just popped the paper bag, which would explain scaring the cat. But even if he’s got a whole lot of paper bags for the kids later, I don’t think that sound would be very scary for most kids. (And I’d expect to have seen some indication of a sound effect.)
I assumed he scared the cat by interupting its litter session to collect droppings.
This all presumes early trick or treaters are a recognizable problem.
Whatever it is, it certainly scared Ludwig.
I don’t think he did anything explicit to scare Ludwig. It’s just whatever scary decoration he put up frightened the cat and Luddy’s reaction is proof that’s it’s really good. Cats can react very strongly to something unusual suddenly showing up in a place they consider familiar. If this were a couple of years ago, I’d suggest it might be a cucumber. (There was a thing back then about people sneakily putting cucumbers behind cats who then freaked out. Kind of like disappearing from behind a curtain is right now.)
Maybe the cat was in the bag, and he let the cat out of the bag.
Arlo’s comment in panel 2 has me stumped. I guess he’s trying to be funny?
I think quite the opposite, I think he’s being very literal: he doesn’t like all the build up, he only likes Halloween on Hallow e’en. (And we’re supposed to understand that for this multiday arc, it’s all taking place on October 31st… (it was a multiday arc, wasn’t it?))
I wish it hadn’t been drawn as though Ludwig’s tail was on fire. Not funny. Not funny at all.
Comics I Don't Understand 
My guess is a scary mask. Arlo might spook the cat, but I’m pretty sure he wouldn’t feed him (or kids, for that matter) something to provoke that reaction.
He could have just popped the paper bag, which would explain scaring the cat. But even if he’s got a whole lot of paper bags for the kids later, I don’t think that sound would be very scary for most kids. (And I’d expect to have seen some indication of a sound effect.)
I assumed he scared the cat by interupting its litter session to collect droppings.
This all presumes early trick or treaters are a recognizable problem.
Whatever it is, it certainly scared Ludwig.
I don’t think he did anything explicit to scare Ludwig. It’s just whatever scary decoration he put up frightened the cat and Luddy’s reaction is proof that’s it’s really good. Cats can react very strongly to something unusual suddenly showing up in a place they consider familiar. If this were a couple of years ago, I’d suggest it might be a cucumber. (There was a thing back then about people sneakily putting cucumbers behind cats who then freaked out. Kind of like disappearing from behind a curtain is right now.)
Maybe the cat was in the bag, and he let the cat out of the bag.
Arlo’s comment in panel 2 has me stumped. I guess he’s trying to be funny?
I think quite the opposite, I think he’s being very literal: he doesn’t like all the build up, he only likes Halloween on Hallow e’en. (And we’re supposed to understand that for this multiday arc, it’s all taking place on October 31st… (it was a multiday arc, wasn’t it?))
I wish it hadn’t been drawn as though Ludwig’s tail was on fire. Not funny. Not funny at all.
today’s comic explains it https://www.gocomics.com/arloandjanis/2018/10/30?ct=v&cti=1471193 and Minor Annoyance had it.
Chak, to me it just looked like an exaggerated motion blur. Also, here’s Frank Hayes singing his “Never Set the Cat On Fire”: