I butterflied a whole dorade but removed the head. It's now sitting in the freezer waiting for a soup or stock. Does anyone know a more interesting or creative use for fish heads? 
Mold doesn't grow that fast in a refrigerator overnight. I'd bet some ingredient reacted with your baking powder or baking soda. I've seen something similar before in three separate circumstances:   instant oatmeal that turned blue when mixed with iodine-treated water (day after day on a camping trip); garlic mixed with yogurt turns blue when heated; blueberry pancake batter turned green overnight.  