Kappaphycus alvarezii, a seaweed known for its carrageenan production, is extensively cultivated in the Philippines, providing livelihoods to fishing communities and generating revenue through exports of dried Kappaphycus and semi-refined carrageenan. Unfortunately, there has been a recent decline in seaweed production attributed to outbreaks of 'ice-ice' disease in seaweed farms. To understand the disease better, this study aimed to document the morphological changes and analyze the pigments and bioactive metabolites of 'ice-ice' disease-infected K. alvarezii. The researchers induced the disease in K. alvarezii thalli under various laboratory conditions involving different salinity (25, 30, 35 ppt) and temperature (25˚C, 27˚C, and 25˚C) levels. The findings revealed that necrosis at the apical tips of K. alvarezii thalli started on the second day of disease induction, particularly in thalli incubated in 25 ppt salinity, regardless of temperature. By day 8, the infection reached degree 5, where 81-100% of the thallus was affected. The infected thalli showed slimy secretions, bleaching, softening, and fragmentation. As the 'ice-ice' disease progressed, the concentration of pigments (chlorophyll a, total chlorophyll, and carotenoid) decreased, and the number of dead cells increased. The concentration of chlorophyll a, total chlorophyll, and carotenoids dipped significantly, while the percentage of dead cells rose to 52% in thalli with 81-100% infection. Bioactive metabolites such as chitin, pectin, and suberin were detected in both uninfected thalli and those with degree 4 (61-80% of thalli infected) infection, but not in thalli with more than 80% infection. Histochemically, 'ice-ice' disease is characterized by decreased chlorophyll a concentration, cell and tissue necrosis, and loss of bioactive metabolites. To address the issue, the study recommends developing a management protocol for 'ice-ice' disease in K. alvarezii based on the information gathered. Additionally, the researchers suggest conducting further research on enhancing photosynthetic pigments of seedstocks before planting them in the field.