In the 2012 Teenage Mutant Ninja Turtles series, the alien species behind the Kraang is the Utroms.
The Utroms are a species of intelligent alien beings whose biology and technology are connected to the Kraang presence in the 2012 continuity. Their influence forms part of the science-fiction side of the series, alongside mutagen experiments, alien technology, and interdimensional threats.
The name can be confusing because “Kraang” is used both for individual characters and for the broader alien force. The Utroms are the species-level identity, while the series uses Kraang terminology for its antagonistic alien figures and organization.
Other franchise versions use Utroms differently or feature separate alien groups, including the Triceratons. This answer is specifically tied to the 2012 Nickelodeon animated continuity.