The Test-Tube Witch is the ghost of a human witch.
The ghost of this impish witch, mixing her potions with chemical precision (and the test-tubes to match) is one of the ghostly residents of the Haunted Mansion. She and others of the Mansion's witches are collectively referenced in one verse of Madame Leota's incantation, "Wizards and witches, wherever you dwell".
Behind the scenes
The Test-Tube Witch was a possible character for the Haunted Mansion ride, sketched by Marc Davis in the Summer of 1964.
His sketch of the character was finally released to the public in September of 2019 in the book Marc Davis In His Own Words.