The old and famous Symbol font is in no way intended to support the Greek language. However, as it provides a basic subset of its alphabet (without diacritics), you may have to deal with imported texts that mimick Greek letters using the Symbol charset. Deciphering such content is a tedious task because this font is not Unicode-compliant and relies on Latin characters: a encodes α, b encodes β, g encodes γ, and so on.

In InDesign, chances are that your main document fonts support the Greek alphabet, or at least the tiny set of glyphs offered by the Symbol font. In such case, you may have some imported text mixing exotic Symbol characters with a fine-tuned paragraph font,

Lamentable intrusion of Symbol's glyphs…

while all desired glyphs are in fact present in your destination typeface:

…but the Glyphs panel shows that we already have all needed in the current font!

Our goal is to remove the Symbol font, but it's not like we had just to apply a character style, because the underlying characters do not match their expected Unicode codepoint. The following script performs the desired conversion:

// Translate symbol letters into actual Greek characters.
// (Select the target text range before you execute this script.)
// String.replace callback.
   return (q=callee.MAP).hasOwnProperty($0) ? q[$0] : $0;
}).MAP =
// Map letters from the Symbol font to Greek letters.
   "a":  "α",      //alpha
   "b":  "β",      //beta
   "g":  "γ",      //gamma
   "d":  "δ",      //delta
   "e":  "ε",      //epsilon
   "z":  "ζ",      //zeta
   "h":  "η",      //eta
   "q":  "θ",      //theta
   "i":  "ι",      //iota
   "k":  "κ",      //kappa
   "l":  "λ",      //lamda
   "m":  "μ",      //mu
   "n":  "ν",      //nu
   "x":  "ξ",      //xi
   "o":  "ο",      //omicron
   "p":  "π",      //pi
   "r":  "ρ",      //rho
   "s":  "σ",      //sigma
   "t":  "τ",      //tau
   "u":  "υ",      //upsilon
   "f":  "φ",      //phi
   "c":  "χ",      //chi
   "y":  "ψ",      //psi
   "w":  "ω",      //omega
   // ---
   "j":  "ϕ",      // phi_symbol
   "v":  "ϖ",      // pi_symbol
   "¡":  "ϒ",      // upsilon_hook_symbol
   // ---
   "A":  "Α",      // Alpha
   "B":  "Β",      // Beta
   "G":  "Γ",      // Gamma
   "D":  "Δ",      // Delta
   "E":  "Ε",      // Epsilon
   "Z":  "Ζ",      // Zeta
   "H":  "Η",      // Eta
   "Q":  "Θ",      // Theta
   "I":  "Ι",      // Iota
   "K":  "Κ",      // Kappa
   "L":  "Λ",      // Lamda
   "M":  "Μ",      // Mu
   "N":  "Ν",      // Nu
   "X":  "Ξ",      // Xi
   "O":  "Ο",      // Omicron
   "P":  "Π",      // Pi
   "R":  "Ρ",      // Rho
   "S":  "Σ",      // Sigma
   "T":  "Τ",      // Tau
   "U":  "Υ",      // Upsilon
   "F":  "Φ",      // Phi
   "C":  "Χ",      // Chi
   "Y":  "Ψ",      // Psi
   "W":  "Ω",      // Omega
   // ---
   "J":  "ϑ",      // theta_symbol
   "V":  "ς",      // sigma_final
function run(  s,a,t)
// Main routine.
   // Checkpoint.
   // ---
   ( && (s=s[0]);
   if( !s || !('textStyleRanges' in s) ) return 0;
   // Constants.
   // ---
   const ST_C = +StyleType.CHARACTER_STYLE_TYPE;
   const OT_C = +OverrideType.CHARACTER_ONLY;
   const RE = /./g;
   const RF = $.global.REPLACE;
   // Loop.
   // ---
   a = s.textStyleRanges.everyItem()
   while( t=a.pop() )
      if( !t.textHasOverrides(ST_C,false) ) continue;
      t.contents = t.contents.replace(RE, RF);
app.doScript(run,void 0,void 0,UndoModes.ENTIRE_SCRIPT, "Symbol2Greek");

This very simple snippet can be adjusted to other fonts and re-encoding schemes. It doesn't explicitly look for the Symbol font. Instead, it detects style ranges that have character style overrides and applies the translitteration to those specific parts. Just select the words or lines that need to be fixed then run the script.

Result of executing the script on a text frame.

If your text only contains Symbol overrides, you can select the parent frame itself, but it is safer to use this tool locally. (Anyway, undo is available in case of error.)