"Indigestible Gruel" Monster #3 from Monsters of the Holiday Feast! (Patreon)

Or can I go with Undigestible Gruel? Spell check has been telling me that it is not a word, (and telling me that I am bad at spelling in general and should feel bad)  ....but I feel like "undigestible" ought to be a word. I know I've read it somewhere. Must I be a slave to the machines?  

What do you think? Indigestible Gruel or Undigestible Gruel?