We don't have synonyms for marsh mallow yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are marshmallow, marsh-mallow and marsh mallows.
(n) :
A European species of plant in the mallow family, Althaea officinalis, historically used medicinally and the source of what became the modern marshmallow confection (which no longer contains the plant).
(n) :
A European species of plant in the mallow family, Althaea officinalis, historically used medicinally and the source of what became the modern marshmallow confection (which no longer contains the plant).