Question about the xenomorph in Alien

You're comparing two different things - how physically strong a material is, as compared to how reactive it is with acids.

Most metals react with acids to one degree or another, so it's no surprise that Alien 'blood' does. But there are materials that don't - glass, for instance, doesn't react with most acids even though it's much physically weaker than a metal like iron.

So Xeno skin being acid-proof doesn't really say anything about how strong it is, it just says that it doesn't react with acid.

As to how strong Xeno skin is, pretty strong. The pulse rifles we saw used against the Xenos in Aliens were firing 10mm explosive tipped bullets, a "light armour piercing" round. So Xenomorph skin could literally be as hard as thin steel and still be ripped apart by those bullets. Certainly stronger than something like a car door, say.