These are the automatic subcircuit labels for Multiplexers. You can either assign some better labels to them as a second argument:
mplexerz1 =
QuantumCircuitOperator[{"Multiplexer", "RZ"[a1], "RZ"[a2], "RZ"[a3],
"RZ"[a4]}, "Multiplexer"]
Or specify an option for subcircuits not to show any label:
mot["Diagram", "SubcircuitOptions" -> "ShowLabel" -> False]