I've received a rhizo. with dark leaves marred by [fertilizer?] whitish stains that don't come off with plain water rub or vegetable soap soak. Anything else to try?
White vinegar, maybe. You might dilute it before you try it.
If it were me, I would have a go at it with very mild baby wipes, they have moved stains on plants for me before, and if they don't hurt a baby I would imagine they're fine for any plant.