How do I get past poisonous snails?

Solution 1:

As you've found the mushrooms they drop are useful for passing their brethren but something I often bring is a fireworks launcher, you can lock on and both kill and cook them in one shot for a quick health pick-me-up. If you'd rather have the mushroom then any ranged attack that isn't fire based will work equally well.

Solution 2:

I decided to take a risk and run into the cloud and punch the snails. Apparently the mushrooms that sprout from their bodies give you poison immunity for a full 30 seconds.

Thus, it looks like the best way to bypass these snails is to just run up to them and punch them.

In case you get poisoned in the process, mashing the x button after being poisoned will cause you to vomit. When you vomit sparkles, it means you've been cured of poisoning. If you vomit blood, you're still poisoned.