There have been a bunch of capsaicin stories here, and with this month's challenge there are likely to be more, and they keep bugging me. So I thought I'd try to set the record straight.
Capsaicin is not corrosive. It is not a general-purpose poison. As a general rule, it has no effect on biological tissues. It is a secondary messenger mimic.
What does that mean? When mammalian cells are destroyed, their last act is to release substances called "endovanilloids". These substances disperse through the body until they reach the right sort of nerve cell, at which point they trigger a burning sensation. This mechanism allows you to feel burning even after the on-fire tissue is destroyed. They also trigger some immune/healing responses.
Capsaicin is an endovanilloid mimic. If it hits those nerve cells, it triggers the same response. The pain is an illusion -- the tissue isn't actually damaged -- but it feels the same. Sometimes the nonneural receptors can trigger enough bogus healing attempts to cause damage, but that's not a direct thing.
On Earth, only mammals use this specific chemical messenger system. Birds can eat jalapeño peppers like they're nothing because they are nothing. For aliens to be effected would require them to be extremely human-like.
(Alternatively, they might use the same receptor for a different purpose -- they get high off capsaicin they way we do off THC, and are shocked to learn why we bred such strong drugs! That could be a story...)
The bird vs mammal thing is probably why capsaicin evolved. Pepper seeds are usefully spread by birds, but destroyed by mammals, so the plant needed a selective defense.
It does say something about humans that when we encountered a plant which protects itself against us by specifically tricking our bodies into thinking they're on fire, our response was to flavor our food with it, to cultivate it, and to breed it for greater potency. But what it says isn't that we're supremely chemically tough -- it says that we're insane.