Yes, CALTROP is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, L 1, T 1, R 1, O 1, P 3.

Is it a valid word?

CALTROP is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

tropical annual procumbent poisonous subshrub having fruit that splits into five spiny nutlets; serious pasture weed
a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits
Mediterranean annual or biennial herb having pinkish to purple flowers surrounded by spine-tipped scales; naturalized in America

Also: devil's weed, tribulus terestris, water chestnut, water chestnut plant, star-thistle, centauria calcitrapa.