  We present a systematic study of the nematic fluctuations in the iron
chalcogenide superconductor Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ ($0 \leq x \leq 0.53$)
using the elastoresistivity technique. Near $x = 0$, in proximity to the
double-stripe magnetic order of Fe$_{1+y}$Te, a diverging $B_{1g}$ nematic
susceptibility is observed. Upon increasing $x$, despite the absence of
magnetic order, the $B_{2g}$ nematic susceptibility increases and becomes
dominant, closely following the strength of the $(\pi, \pi)$ spin fluctuations.
Over a wide range of compositions ($0.17 \leq x \leq 0.53$), while the $B_{2g}$
nematic susceptibility follows a Curie temperature dependence (with zero Weiss
temperature) at low temperatures, it shows deviations from Curie-Weiss behavior
for temperatures higher than $50K$. This is the opposite of what is observed in
typical iron pnictides, where Curie-Weiss deviations are seen at low
temperatures. We attribute this unusual temperature dependence to a loss of
coherence of the $d_{xy}$ orbital, which is supported by our theoretical
calculations. Our results highlight the importance of orbital differentiation
on the nematic properties of iron-based materials.
