You probably need to ask Aphex Twin. It could even be a typo, since he has also called himself Blue Calx (not Blue Clax). However, his spelling is, shall we say, creative? drukqs anyone?
Breaking news: According to that venerable and reliable source, Wikipedia:
“Calx is a residual substance, sometimes in the form of a fine powder, that is left when a metal or mineral combusts or is calcinated due to heat.
Calx, especially of a metal, is now known as an oxide. According to the obsolete phlogiston theory, the calx was the true elemental substance, having lost its phlogiston in the process of combustion… UK Electronic music artist, Aphex Twin (Richard David James) named some of his tracks after different coloured Calx (green, yellow and blue).” (http://en.wikipedia.org/wiki/Calx)