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. The gene discussed is ALOX5; the disease is cancer.