When she and her partner, Jonathan Cohen, who painted the mural, demanded that they stop, Ms. Flageul said the workers indicated that they were taking orders from the restaurant owner across the street.

“This is more than a painting of a mural — it’s a shrine,” Ms. Flageul said.

Ms. Flageul and Mr. Cohen, better known by his tag name Meres One, are no strangers to graffiti preservation. Both were players in the landmark case involving 5Pointz, a complex in Long Island City, Queens, that housed the work of 21 graffiti artists, including Mr. Cohen’s. Five years after about 45 illustrations were whitewashed by a new developer, a federal judge fined him $6.7 million for destroying public art.

On July 10, the rumor spread even more after Price’s widow, Bernadette Price, posted on Facebook about the mural being in danger. She also created an online petition to save the mural, which surpassed its 15,000 signature goal.

But a story that began with concern over the preservation of a neighborhood’s cultural heritage rapidly devolved into threats and hate speech.

“If that mural goes down, the restaurant gonna go down next,” read one comment on Ms. Price’s post, accompanied by flame emojis.