You also need `InputForm` if you want to convert \\[Pi] to Pi

If you want parentheses

    2 \[Pi]/5 // ToString[#, InputForm] & // StringReplace[#, "/" :> "--"] &

> "(2*Pi)--5"

Else

    2 \[Pi]/5 // ToString[#, InputForm] & // 
     StringReplace[#, {"/" :> "--", "(" :> "", ")" :> ""}] &

> "2*Pi--5"