FCGV[x] is a FeynCalc global variable, i.e. a container for string variables that allows to introduce new variables without polluting the Global context of Mathematica.
Use the rule FCGV[s_] :> ToExpression[s] if you want to convert the string x to a symbol with the name x.
FCGV["x"]\text{FCGV}(\text{x})
FCGV["x"] // InputFormFCGV["x"]