Phytochemical investigation of the brown algae S. cinereum with the guidance of LC–HRESIMS dereplication afforded two new phenolic derivatives 12 and 13, along with the known 19, which exhibited moderate in vitro antiproliferative activity against HepG2, MCF-7, and Caco-2 cancer cell lines and considerable selective inhibition toward 5-LOX over 15-LOX. This evidence concerns the gene ALOX15 and cancer.