Lewis & Short

alga, ae, f. [from ligo, qs. alliga, as binding, entwining, Van.], sea-weed, comprising several kinds, of which one (Fucus vesiculosus, Linn.) was used for coloring red, Plin. 26, 10, 66; 32, 6, 22, § 66. Freq. in the poets, Hor. C. 3, 17, 10; Verg. A. 7, 590; so Mart. 10, 16, 5; Val. Fl. 1, 252; Claud. Ruf. 1, 387. In prose, Auct. B. Afr. 24 fin.
Hence also for a thing of little worth: vilior algā, Hor. S. 2, 5, 8: projectā vilior algā, Verg. E. 7, 42.