Ulva sp. (green seaweed) often proliferates explosively and piles up in shallows. This phenomenon is called “green tide,” caused by increased nutrient flow into an enclosed sea area. Although green tide is one of the environmental problems in coastal areas, Ulva sp. can be regarded as carbon-neutral and therefore can serve as an abundant feedstock for renewable energy. Methane fermentation is one of the suitable techniques for converting such seaweed into energy. Digestate from methane fermentation is normally used as fertilizer; however, it is ends up being treated as wastewater due to limited spaces in urban areas. This paper proposes that the digestate from methane fermentation using mixed biomass (Ulva sp. and food waste) can be applied to the cultivation of Pyropia yezoensis (edible laver seaweed, nori in Japanese), which has recently suffered from decolored phenomena because of decreasing nutrients. The absorption of nutrients and the color recovery of nori were investigated in laboratory-scale experiments based on comparison with artificial seawater. The results highlight that the significant absorption of nutrients and color recovery occurred because of digestate utilization, indicating its positive effects on decolored nori. In addition, the experiments found that not only NH4-N, but also other substances such as trace metal related to Ulva sp. can influence such effects. The findings clearly indicate that digestate can be used in the sea and that the suggested multiple uses of digestate would increase the value of digestate.