Efron captioned the photo: "Just for fun".
"It's not about how his dreads look it's about cultural appropriation which doesn't fly even if it was 'just for fun'... whatever that means. Zac Efron has had a special place in my... heart for years but that doesn't mean he gets a free pass," one user wrote.
Another said, "What in the cultural appropriation is going on?"
"I have really seen it all," some user said.
There were also a few who defended "The Greatest Showman" actor, saying people are blowing things out of proportion.
"It is just damn hair. Geesh," one said.
"Looking good," another user said.
Another Instagrammer took a history class on the photo-video-sharing site, highlighting that apart from African-American tradition, dreadlocks have a presence in other cultures around the world.
"Vikings had dreads and so did the ancient Egyptians... You Black People in the states are the most hypocritical and uneducated people in the world. What about Black women with straight hair wigs or blond hair is that not racist to them. Hahahah (sic)," he said.
Locks have been worn for various reasons in each culture: as an expression of deep religious or spiritual convictions, ethnic pride, as a political statement and in more modern times, as a representation of a free, alternative or natural spirit.