Look, whoever thinks that Predator armor is not acid resistant knows nothing about their technology. Their whole technology is built to counter the Aliens. The Xenomorph is their most prized prey, so you'd think, that logically, considereing how smart the Predators have proven to be, that they would construct equipment to counter an Alien, especially their acid blood. Heck, if the armor, made of an alloy called xerbinium by the way, wasn't acid resistant, why would they bother wearing any during an Alien hunt if they knew that their acid blood would melt it anyway. They've been fighting the Aliens for thousands of years, so to say that they dont know about the acid blood is absurd. I'd go more with the theory that his helmet got knocked off during a duel and the facehugger took advantage, but it'd be quite impossible for acid blood to melt xerbinium, the Yautja use it for a freaking reason!