We don't have synonyms for button grass yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are cotton grass and bottom grass.
(n) :
(chiefly Australia) Any of several grasses with round or globular flower heads; especially Gymnoschoenus sphaerocephalus, a tussock-forming sedge especially of western Tasmania, forming distinct plains; and Dactyloctenium radulans, an annual grass of mainland Australia.
(n) :
(chiefly Australia) Any of several grasses with round or globular flower heads; especially Gymnoschoenus sphaerocephalus, a tussock-forming sedge especially of western Tasmania, forming distinct plains; and Dactyloctenium radulans, an annual grass of mainland Australia.