It may be a silly question but I ask anyway because I feel like this must have happened to many people. So I often buy red snapper (真鯛) in my local groceries. But sometimes in certain part of the fish, the flesh got a greenish stain on it such as shown in the picture below. Does this indicate the deterioration of the fish? But I always make sure that the fishes I take from the shelf are those from the most recent date (i.e. the most fresh). This had happened from different groceries.