HSPICE command naming weirdness
This is standard SPICE syntax and I suspect it is documented in many places. Instance names for MOSFETs must begin with M and instance names of subcircuits must begin with X. Voltage sources start with V, resistors start with R, and so on. The first letter tells SPICE how to parse the rest of the line.
It gets tricky when a MOSFET is actually described using a subcircuit rather than directly with a MOSFET model, but if it's a subcircuit then it needs to start with an X.