Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
neptūnĭa, ae, f., a plant, App. Herb. 57.
Neptūnĭus, a, um, adj. [Neptunus], of or belonging to Neptune, Neptunian; poet. for sea-, marine: Neptunia Troja, surrounded with walls by Neptune, Verg. A. 2, 625; 3, 3: proles, id. ib. 7, 691: heros, i. e. Theseus, as the son of Neptune, Ov. H. 4, 109; id. M. 9, 1: dux, i. e. Sex. Pompeius, who called himself the adopted son of Neptune, Hor. Epod. 9, 7 Schol. Cruq.: Aetneae Neptunius incola rupis, i. e. Cyclops, the son of Neptune, Tib. 4, 1, 56: cuspis, the irident, Luc. 7, 147: loca, i. e. the sea, Plaut. Mil. 2, 5, 3; so, arva, Verg. A. 8, 695: pistrix, i. e. marina, Cic. Arat. 440.