Yeah. the two are actually quite similar. They're both spread by spores, and root through the body after they grow. Some plants even disperse those parasitic spores, and the spores can make those certain types of plants grow XD
Ah. In this picture, Amaru is utilizing his "Black" transformation, which is something any Necro can do. Simply put, he's allowed the parasite to exit his belly by unhinging his jaw and regurgitating it. The parasite takes up most of the space in his belly, which is why he's looking so skinny. Because the parasite is moving it recalls it's tendrils, which help move his limbs and tail so Amaru falls limp and unresponsive. In this state, the parasite basically takes over the body, showing the actual dependence that hosts have on their parasites.
Anyway when it's free, the parasite can shoot homing worms to disorient, infect and stun it's target so that the host can defeat it. When its mouthparts are flared and exposing it's mouth, it's getting ready to shoot.