Ссылка на статью: The Giraffe. Малькольм Линдси.
"грот" definately stands here for "grot", "grotto", a small cavern.
It'd be no correct to say in Russian "hide himself (прятаться) in a marble mainsail", even for the Poet. Correct: turn or change into, become, grow, etc. (оборачиваться, превращаться, становиться). Thus, your translantion of the word "грот" is right, and the assumption made is wrong.