It used acid to melt the faceplate.
I believe the one that got Ripley in Aliens3, used its acid to melt into her suspended animation ‘coffin’ and facehug her. The reason we don’t see the facehuggers use that technique often, is because it’s only needed when the host is protected by some barrier, such as a helmet.
In most of the films, once the initial person is infected, the aliens are eventually brought into an environment that doesn’t require their hosts to be wearing helmets or the victims are pasted onto a wall and exposed; so there’s no need for the facehuggers to use their acid as a barrier remover…because there isn’t one.