It`s not about how resistant it is, the blood is only acid for a few seconds and that`s how many seconds it took for the scar to be made after the whole tooth-like-mandible was disolved. We have no idea how hard predator flesh is compared to the floor of the Nostromo and it doesn`t really matter. Acid doesn`t attack like a maniac whenever it can, it just reacts with what comes in contact with it.