It's just two "viscous fluids". No further scientific explanation is offered in any of the comics (or supplementary materials) regarding the composition of the material or liquid involved.
RORSCHACH:
Rorschach is the replacement for The Question, and draws his
name from the famous psychiatrist's blot-test. While he wears an
assortment of street clothes rather than a costume, his entire body is
encased in a thin, hollow membrane, semi-porous and incredibly thin.
This contains two viscous liquids, one black and the other white. The
liquids flow and change according to variations in pressure and body
temperature, making the black and white blots run over his body in an
ever-changing design. His face is a white blank marked only by a
constantly altering Rorschach blot.
The black-and-white motif ties in with the character's very hard-edged
moral philosophy, and is also used by the character himself as a
metaphor for the ambiguity of life. Each man must read his own fate
into the pictures conjured by the neutral Rorschach blot, and judge
himself accordingly.
Watchmen: Absolute Edition

Watchmen #06

Watchmen: Absolute Edition