The Sanitized creatures are digitally generated at will by computers controlled by Tartar. They are virtual, imperfect (green/blue) recreations of real species (Octarians mainly) given physical presence by NILS/Kamabo Co's weird distortion of physical space.
To be able to recreate an avatar of a species, a sample of the subject's tissue and blood is taken for DNA purposes, and the subject is forcibly immersed in a gel that systems generate the 3D data with.
The Metro, when the test stations were being constructed, broke into one of the Octarian habitation caves. A team was sent to investigate - ‘sanitized’ Octolings in the image of the team members soon started appearing in that area to attack, and the cave’s inhabitants were relocated and the cave sealed off.
As a result, this singular Octarian cave is held under the influence of the Sanitized, and is affected by NILS' electromagnetic field generator.
The substance that held Agent Three under Tartar’s control and that oozes from Tartar’s ‘true form’ does not sanitize (going against fanon). This substance is a means for Tartar to interface with (or control) organic life forms, as a byproduct of his experiments.