It can be both.

Normally its gooey.

No, "gooey" is not an adverb. It is an adjective used to describe something as soft, sticky, and thick. Adverbs typically describe how an action is done.

I'm not sure what a gooey alien is or even what you would feed him. But if there is anything gooey involved put a bib on him first.

Nerves are white, not green gooey lines so probably not.

Slop fits the description - it is a soft, wet mess that can be considered gross and gooey.