Yes, it's the melamine resin foam itself.
BASF Group: Spring cleaning ... as if by magic
"What looks under the electron microscope like foam on bath water is actually a finely structured three-dimensional network consisting of extremely slender and therefore flexible plastic filaments. The large, almost freely accessible surface produced by this airy structure
binds the abraded particles of dirt and contributes to the eraser's astounding grime-removing capabilities." [emphasis mine]
"Only surfaces liable to scratching and varnished surfaces should not be cleaned with the "magician.'"