From Destinypedia, the Destiny wiki
Radiolaria, referred to by some Guardians, and Clovis Bray I himself , as "Vex Milk", are microscopic silica-based organisms that make up the biological component of Vex constructs. Massive concentrations of radiolaria are often found on planets transformed into machine worlds.
In the state preceding the universe as commonly understood, the Vex first existed as abstract patterns, which escaped to the universe when the Light and the Darkness began warring with one another. These patterns were able to influence the chemistry within the liquid cores of comets, creating the precursors of a physical substrate to house their consciousness. These comets then rained down on lifeless planets, and the first radiolaria evolved from the chemicals within.
Shortly prior to the Collapse, both the Ishtar Collective and Clovis Bray I conducted detailed studies of Vex radiolaria. In addition to cataloging their physical characteristics, Clovis I also discovered that exposing radiolaria to the paracausal force he called "Clarity" produced a substance that he called "Alkahest" which, when introduced to the hardware of an exomind, miraculously solved the "loop/billboard/crash" problem that had until that point unavoidably plagued early exominds.
Vex radiolaria are microscopic organisms that are typically suspended in a briny white, luminescent fluid. They are found on planets transformed by the Vex, such as Nessus. They control the mechanical chassis of the Vex from a container usually found within their chests. On some Vex units this "brain" is located elsewhere; for example, behind the eye on Harpies or in the abdomen on Cyclopes.
Vex milk is non-Newtonian, highly conductive, and noncompressible. Its viscosity and surface tension are variable: it can form a resistant membrane, or climb the walls of a container like a superfluid. The milk can also store kinetic energy in zero-viscosity vortices, essentially liquid flywheels. This allows them to, for example, retain the motion of something stirring them so that they may use it in the future.
When radiolarian fluid comes into contact with non-Vex matter, it will attempt to turn the host into a Vex unit. Examples include Kabr, the Legionless, who drank from an Oracle and was transformed into a Vex, and Asher Mir, who lost an arm to radiolarian "infection". Another example comes from stepping in radiolaria in-game, as it will slowly damage players. At times it even possesses a hallucinogenic and entheogenic effect.