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 marshmellow.
(n) :
Alternative spelling of marsh mallow. [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) :
Alternative spelling of marsh mallow. [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).]