You can tell right away it's acid. If you look carefully in the 3rd panel you can see bubbles coming off partf of her. It's mroe obvious, becasue if you notice she is wearing an anklet, the bottom pic shows the planty thing spitting it out.
Everyone who says this isn't funny needs to watch the NatGeo Channel once in a while. Pretty much all living things eat other living things. This is just a fantasy twist.