I trust, OP, you appreciate the… clashiness of your request. Even if Rumi actually did write some verses in Greek.
I guess, καυχῶμαι μουσουλμάνος σουφιστὴς ὤν. You won’t find “Mussulman” in any Koine texts, but you certainly find the adjective in the 12th century. You won’t find “Sufi” in Byzantine Greek either, and Ottoman Greeks would far likelier have referred by name directly to the Mevlevi Order they were familiar with; but I don’t think the modern reference to “Sufist” would be out of place.
Well, no more than the whole sentence would be, really. 🙂