You will need to throw out your ferment and source fresh grains. There should be no discolouration or mould on the layer exposed to the air. Sometimes the top layer can look a little dry, and if you use unhomogenised/fresh milk, the cream at the top can have a healthy tinge of yellow. You can stir this in. If there is any other discolouration or a rank smell - throw it away and source new grains.