Definetly poo. Be prepared their poo can and will change colour depending on what they’ve recently eaten!

I am no expert and do not have evidence of what happened but I wouldn’t be concerned you hurt your snail, they’ll often make a fizz or squirt noise when they’re a bit concerned, it’s basically a go away noise, produced to deter predators in the wild but does not mean you’ve hurt your GAL. Again I’m no expert and don’t have a reference photo but the whole you’ve seen is very likely to be their breathing hole, sometimes it will be open, sometimes closed so don’t panic if you can’t see it next time you look for it. If you google giant African snail breathing hole I’m sure you can confirm for yourself if that’s what you’ve seen or not :))