Lesson: [s1e9] The Anatomy

Noah Griffith & Daniel Stewart; Teleplay by Mark Verheiden.

He is a mass of swamp vegetation that "dreamed" it was Alec after absorbing the scientist's memories and DNA following the lab explosion. [S1E9] The Anatomy Lesson

To confirm this, Swamp Thing later retrieves Alec Holland’s actual decayed corpse from the water, realizing he is merely a plant mimicry. Subplots & Transformations Noah Griffith & Daniel Stewart; Teleplay by Mark Verheiden