This is a textual inversion embedding to add makeup/mascara running from the eyes, as often seen in fellatio/irrumatio images, but can be used elsewhere too. Use corneo_runny_makeup to activate it.
It can come out pretty heavy by default (i.e. a LOT of black mascara running down the face quite thick) so you may want to tone it down by de-emphasising the tag such as (corneo_runny_makeup:0.9).