  We report neutron scattering measurements, which reveal spin-liquid
polymorphism in a '11' iron chalcogenide superconductor, a poorly-metallic
magnetic FeTe tuned towards superconductivity by substitution of a small amount
of Tellurium with iso-electronic Sulphur. We observe liquid-like magnetic
dynamics, which is described by a competition of two phases with different
local structure, whose relative abundance depends on temperature. One is the
ferromagnetic (FM) plaquette phase observed in the non-superconducting FeTe,
which preserves the C$_4$ symmetry of the underlying square lattice and is
favored at high temperatures. The other is the antiferromagnetic plaquette
phase with broken C$_4$ symmetry, which emerges with doping and is predominant
at low temperatures. These findings suggest a first-order liquid-liquid phase
transition in the electronic spin system of FeTe$_{1-x}$(S,Se)$_x$. We thus
discover remarkable new physics of competing spin liquid polymorphs in a
correlated electron system approaching superconductivity. Our results
facilitate an understanding of large swaths of recent experimental data in
unconventional superconductors.
