Users on the social media site were quick to point out the error.
“Hey Super Space Genius, that’s the moon in a total lunar eclipse,” said Upulie Divisekera, an Australian scientist, in reply to the post.
The SpaceX founder appeared to admit the mistake in subsequent tweets joking about the faux pas.
“Moon too,” Mr Musk said, using laughing emojis in a separate reply.
“Have you gone moon first on us?” another user asked.
“For sure moon 1st,” Mr Musk replied.
“As it’s only 3 days away & u don’t need interplanetary orbital synchronization.”
Some of Mr Musk’s followers wondered if his tweet was mocking Donald Trump.
The president recently said NASA should “be focused on the much bigger things ... including Mars (of which the Moon is a part)”.
It is unclear if Mr Musk’s post was a reference to the US leader’s comments.
The billionaire hopes that humans will one day travel to Mars on a Space X craft.
The company is currently testing its Starship spacecraft, which Mr Musk has previously said could take up to 100 people to the Red Planet.