EKNODINE

The Eknodine are a symbiotic sentient alien species that can live inside of specimens of other species, extending their lifespan. As a defense mechanicsm, they can extend a green eyestalk from their hosts' mouth, which can spray a green cloud which turn a human-size hostile into dust.
Note: There are similarities between this species and the Goa_Uld. and the Hive/Ganglion.
Source: Doctor Who Universe (DW: Amy's Choice)