Aliens (SpongeHenge)

The Aliens appear at the end of the episode "SpongeHenge," looking at the stone sculptures made by SpongeBob.

Appearance

They are purple and have an extra eye on their forehead. They travel in rocket-powered pods and carry their cameras in their mouths.

They live in the 51st century, which is 1000 years after the future in the episode "SB-129."
It's likely that they are unable to breathe underwater, since they have a dome covering themselves.