SMPToSymbol[exp] converts objects of type SMP["sth"] in exp to symbols using ToExpression["sth"].
The option StringReplace can be used to specify string replacement rules that will take care of special characters (e.g. ^ or _) that cannot appear in valid Mathematica expressions. SMPToSymbol is useful when exporting FeynCalc expressions to other tools, e.g. FORM.
SP[p] - SMP["m_e"]^2
SMPToSymbol[%]\overline{p}^2-m_e^2
\overline{p}^2-\text{me}^2